I've discussed this error with several of my students recently, and hope it will benefit other learners of English.

You will hear native English speakers (in all dialects) use the "words" wanna, gonna, gotta, hafta, oughtta, coulda, shoulda, woulda but these are not standard English contractions like it's, he'd, you're and aren't.

What you're hearing is called "assimilation" and it happens in most, if not all, languages. In the case of wanna/gonna/gotta/hafta/hazta, in spoken English the particle to is being assimilated (or slurred) into the preceding modal verb. In the case of coulda/shoulda/woulda, the modal is assimilating the auxiliary verb have. You'll hear the fully articulated, unassimilated form when the speaker wants to convey emphasis ("I've simply got to get this done by 5 o'clock") or formality ("We have to respect the rights of others").

Assimilation is particularly strong in North American English, where the preposition to is commonly pronounced tə.

Why does this matter? These are spoken forms and shouldn't be used in writing -- especially in business, academia, or proficiency exams such as the IELTS or Cambridge. Basically, nonstandard "contractions" are only used to write character dialog ("'You gonna eat that?' she said"), or song titles & lyrics ("I Wanna Hold Your Hand").