Grammatical words are words that help us construct the sentence but they don't mean anything: articles, prepositions, conjunctions, auxiliary verbs, etc.

These words have no stress, and so they are weakened. That weakened form is called "weak form" as opposed to a "strong form", which is the full form of the word pronounced with stress. The strong form only happens when we pronounce the words alone, or when we emphasize them. Weak forms are very often pronounced with a schwa, and so are very weak and sometimes a bit difficult to hear properly.

Sometimes weak forms are easy to spot, because we use contractions in the spelling to show it:

I am French (strong form) I'm French (weak form)

But usually there is no change of spelling, only the pronunciation is different:

But strong form: /bʌt/ weak form: /bət/

Tell him to go strong forms /hɪm/ /tu:/ weak form: /tel əm tə gəʊ/

As you can see, the grammatical words "him" and "to" are unstressed and have a weak form when pronounced inside a sentence.

another example: I would like some fish and chips

strong forms /aɪ wʊd laɪk sʌm fɪʃ ænd tʃɪps/ This version sounds unnatural and, believe it or not, more difficult to understand for a native speaker.

weak forms /ɑ wəd laɪk səm fɪʃ ən tʃɪps/ and we can use weaker forms sometimes: /ɑd laɪk səm fɪʃ ən tʃɪps/ so we can see that the auxiliary verb "would" has two weak forms /wəd/ and /d/

Students who are learning English usually use only strong forms, and they sound very unnatural. English speakers use weak forms all the time, every single sentence is full of them, and students find it difficult to understand because they are not used to them, and very often they don't even know they exist.

Why do grammatical words weaken the way they do. It's all about rhythm. The way English is pronounced makes it necessary to weaken function words so you can keep the rhythm. You can find more about rhythm here or simply watch this introduction video:

If you want to learn and practise weak forms follow these links:

  A video explaining more about  Rhythm & Weak Forms

Pronunciar las formas débiles   una web con explicaciones en español.

Phonetics in British songs we analyse the pronunciation of the British group One Direction and Ed Sheeran to see the weak forms in action as they sing.