Once again, this deserves its own article, but weak forms are incredibly common in English and it is very hard to understand the language if you unaware of them.
Weak forms are forms of many common words (articles, auxiliary verbs, prepositions, conjunctions, etc.) that are pronounced with the ever-present vowel schwa (sometimes known as the neutral vowel). We mainly use the weak form when a word is unemphasised in a sentence, i.e. it is said with a low tone.
Common words with weak forms are: to, for, from, them, are, but… and many more!