If you’re a non-native speaker, one way to improve your pronunciation is to make sure you know how the word is written in the International Phonetic Alphabet (IPA). Wiktionary is a good resource for this, and maybe also Lexico or Cambridge Dictionary. It’s very difficult to correctly guess the pronunciation of an English word based on its spelling. Many non-native speakers don’t do enough vowel reduction and overpronounce vowels that are actually /ə/.
Note that IPA in dictionaries is almost always phonemic and not phonetic, which means that it will not represent things like:
In American English, /p, t, tʃ, k/ are aspirated as [pʰ, tʰ, tʃʰ, kʰ] at the beginning of a stressed syllable, and unaspirated otherwise.† /p, t, k/ are aspirated after /s/ /t/ is often pronounced as [ɾ] between vowels (intervocalically, often even across word boundaries)‡ and as [ʔ] at the end of a syllable, except sequences like /st/ and /kt/ have [t] or [tʰ]. When the speaker is trying to speak quite formally, they may pronounce it as [tʰ], though that sounds a bit stilted to me.
I’m not sure about British English, but I believe that it is similar to American English except that intervocalic /t/ is pronounced [tʰ] or more casually as [ʔ], and never as [ɾ].
In American English, /d/ is also pronounced as [ɾ] between vowels except when beginning a stressed syllable. Again, speakers may sometimes pronounce it as [d] when trying to speak particularly formally or clearly.
“Vowels are [slightly] shortened when followed in a syllable by a voiceless (fortis) consonant. This is known as pre-fortis clipping. Thus in the following word pairs the first item has a shortened vowel while the second has a normal length vowel: ‘right’ /raɪt/ – ‘ride’ /raɪd/; ‘face’ /feɪs/ – ‘phase’ /feɪz/.” For American English, “writer” and “rider” are both pronounced [ˈɹaɪɾɚ] but the /aɪ/ is pronounced longer in “rider”, because /t/ is a voiceless consonant.
“In many accents of English, tense vowels undergo breaking before /l/, resulting in pronunciations like [pʰiəl] for peel, [pʰuəl] for pool, [pʰeɪəl] for pail, and [pʰoʊəl] for pole.” (Wikipedia)
Some dictionaries for American English treat [ʌ] and [ə] as allophones of /ə/. In those dictionaries, /ə/ in stressed syllables should be pronounced [ʌ].
Some (even weirder) dictionaries write /e/ when they really mean /ɛ/ or /eɪ/, and /o/ when the mean /oʊ/.
Many speakers of American English drop the /t/ in unstressed /nt/. E.g., they might pronounce “center” as [ˈsɛnɚ] instead of [ˈsɛntʰɚ].
In American English, /aʊ/ is pronounced /æʊ/
I feel like /aɪ, eɪ, oʊ, aʊ/ are actually pronounced [ai, ei, əu, au].
British English is mostly non-rhotic, but inserts a /ɹ/ between vowels, even across word boundaries.
There’s also Australian English, but I really don’t understand the phonetic rules for that.
There are lots of other phonetic rules such as dark vs light /l/, but as a speaker of American English I don’t really understand them.
† One exception (from Wikipedia): When the consonants in a cluster like st are analyzed as belonging to different morphemes (heteromorphemic) the stop is aspirated, but when they are analyzed as belonging to one morpheme the stop is unaspirated. For instance, distend has unaspirated [t] since it is not analyzed as two morphemes, but distaste has an aspirated middle [tʰ] because it is analyzed as dis- + taste and the word taste has an aspirated initial t.
‡ However, unstressed /tən/ is pronounced /ʔən/. “Curtain” is pronounced as [ˈkʰɚʔən], not [ˈkʰɚɾən], and “button” is pronounced [ˈbʌʔən], not [ˈbʌɾən].
Now that you know the phonetic representation, it’s time to learn how to pronounce the phonemes/phones! /ɹ, ð, θ, ɑ, æ, ɪ, ɛ, ʊ, ʌ/ can be particularly difficult. I would personally focus on /ʌ/ as it’s quite common and pronouncing it as /a/ sounds weirder than common approximations for other phonemes.
Honestly though, I’ve felt that speaking with perfect pronunciation isn’t as important as having the intonation (variation in pitch of the words of a sentence) similar to a native speaker’s. I think the only way to learn a native-sounding intonation is to hear English often.
While this is all true, I can’t remember having trouble with the pronunciation or tone of non-native English speakers I’ve met in EA. I think there’s probably lower-hanging fruit in having native English speakers adjust their behaviour, like this article suggests.
If you’re a non-native speaker, one way to improve your pronunciation is to make sure you know how the word is written in the International Phonetic Alphabet (IPA). Wiktionary is a good resource for this, and maybe also Lexico or Cambridge Dictionary. It’s very difficult to correctly guess the pronunciation of an English word based on its spelling. Many non-native speakers don’t do enough vowel reduction and overpronounce vowels that are actually /ə/.
Note that IPA in dictionaries is almost always phonemic and not phonetic, which means that it will not represent things like:
In American English, /p, t, tʃ, k/ are aspirated as [pʰ, tʰ, tʃʰ, kʰ] at the beginning of a stressed syllable, and unaspirated otherwise.† /p, t, k/ are aspirated after /s/ /t/ is often pronounced as [ɾ] between vowels (intervocalically, often even across word boundaries)‡ and as [ʔ] at the end of a syllable, except sequences like /st/ and /kt/ have [t] or [tʰ]. When the speaker is trying to speak quite formally, they may pronounce it as [tʰ], though that sounds a bit stilted to me.
I’m not sure about British English, but I believe that it is similar to American English except that intervocalic /t/ is pronounced [tʰ] or more casually as [ʔ], and never as [ɾ].
In American English, /d/ is also pronounced as [ɾ] between vowels except when beginning a stressed syllable. Again, speakers may sometimes pronounce it as [d] when trying to speak particularly formally or clearly.
“Vowels are [slightly] shortened when followed in a syllable by a voiceless (fortis) consonant. This is known as pre-fortis clipping. Thus in the following word pairs the first item has a shortened vowel while the second has a normal length vowel: ‘right’ /raɪt/ – ‘ride’ /raɪd/; ‘face’ /feɪs/ – ‘phase’ /feɪz/.” For American English, “writer” and “rider” are both pronounced [ˈɹaɪɾɚ] but the /aɪ/ is pronounced longer in “rider”, because /t/ is a voiceless consonant.
“In many accents of English, tense vowels undergo breaking before /l/, resulting in pronunciations like [pʰiəl] for peel, [pʰuəl] for pool, [pʰeɪəl] for pail, and [pʰoʊəl] for pole.” (Wikipedia)
Some dictionaries for American English treat [ʌ] and [ə] as allophones of /ə/. In those dictionaries, /ə/ in stressed syllables should be pronounced [ʌ].
Some (even weirder) dictionaries write /e/ when they really mean /ɛ/ or /eɪ/, and /o/ when the mean /oʊ/.
Many speakers of American English drop the /t/ in unstressed /nt/. E.g., they might pronounce “center” as [ˈsɛnɚ] instead of [ˈsɛntʰɚ].
In American English, /aʊ/ is pronounced /æʊ/
I feel like /aɪ, eɪ, oʊ, aʊ/ are actually pronounced [ai, ei, əu, au].
British English is mostly non-rhotic, but inserts a /ɹ/ between vowels, even across word boundaries.
There’s also Australian English, but I really don’t understand the phonetic rules for that.
/ə/ is sometimes pronounced /ɪ/ in some contexts in American English; see https://en.wikipedia.org/wiki/Phonological_history_of_English_close_front_vowels#Weak_vowel_merger.
There are lots of other phonetic rules such as dark vs light /l/, but as a speaker of American English I don’t really understand them.
† One exception (from Wikipedia): When the consonants in a cluster like st are analyzed as belonging to different morphemes (heteromorphemic) the stop is aspirated, but when they are analyzed as belonging to one morpheme the stop is unaspirated. For instance, distend has unaspirated [t] since it is not analyzed as two morphemes, but distaste has an aspirated middle [tʰ] because it is analyzed as dis- + taste and the word taste has an aspirated initial t. ‡ However, unstressed /tən/ is pronounced /ʔən/. “Curtain” is pronounced as [ˈkʰɚʔən], not [ˈkʰɚɾən], and “button” is pronounced [ˈbʌʔən], not [ˈbʌɾən].
Now that you know the phonetic representation, it’s time to learn how to pronounce the phonemes/phones! /ɹ, ð, θ, ɑ, æ, ɪ, ɛ, ʊ, ʌ/ can be particularly difficult. I would personally focus on /ʌ/ as it’s quite common and pronouncing it as /a/ sounds weirder than common approximations for other phonemes.
Also be sure to know which syllable of the word is stressed. Also note that some words are pronounced or stressed differently depending on whether it is a noun or a verb (see https://en.wikipedia.org/wiki/Initial-stress-derived_noun#List).
Honestly though, I’ve felt that speaking with perfect pronunciation isn’t as important as having the intonation (variation in pitch of the words of a sentence) similar to a native speaker’s. I think the only way to learn a native-sounding intonation is to hear English often.
While this is all true, I can’t remember having trouble with the pronunciation or tone of non-native English speakers I’ve met in EA. I think there’s probably lower-hanging fruit in having native English speakers adjust their behaviour, like this article suggests.