Use weak forms to sound natural: reduce function words in fast, connected speech.
Reduce function words (articles, prepositions, auxiliaries) when they are not the focus.
Do not reduce a word when it is emphasized (contrast or correction).
Is this word important/new information? If not, can it be a weak form?
💡 Common patterns: to /tə/ (strong /tuː/), for /fə/ (strong /fɔːr/ or /fɔː/), of /əv/ (sometimes /ə/), and /ən/ (strong /ænd/). British vs US note: weak forms occur in both varieties; the main difference is often rhoticity (US usually keeps /r/ in for /fər/, many UK accents may not).
✓I want to go → /aɪ ˈwɒnə ɡəʊ/ (to → /tə/; sometimes want to → wanna)
✗I want TO go (strong /tuː/ every time)
Unstressed 'to' is often reduced; in very casual speech it may merge with surrounding words.
✓a cup of tea → /ə ˈkʌp ə tiː/
✗a cup OF tea (strong /ɒv/ every time)
'of' is frequently reduced in fast speech when unstressed.
✓fish and chips → /ˌfɪʃ ən ˈtʃɪps/
✗fish AND chips (strong /ænd/ every time)
'and' is commonly reduced to /ən/ when unstressed.