"Using "gonna" correctly will help your English sound more natural. "Gonna" is not rude or bad English. It is just the shortening of a phrase that English speakers use very often..." The thing is, every native English speaker knows exactly what proper speech sounds like. So even if we use "gonna" and friends, we will take more care with our language if we need to. Using "gonna" as a learner is a very bad idea. It makes your speech sound sloppy (even unnatural) and it highlights your other mistakes. As well, it gives the message of "I don't really care" when you talk to a native speaker. So there's nothing wrong with speaking English properly.

I think most English speakers would deny that they ever use those words, but actually them all the time without realising it. If you consciously try to say 'gonna' (for example), it's not going to sound natural. Always try to say 'going to', 'got to' and 'kind of'; if you're fluent enough and you're speaking quickly enough, your tongue will say 'na' when you're trying to say 'to' in a rush to speak the next word. Note that native speakers say the 'to' silently with their tongue, but don't end up enunciating the sound because of how quickly they're speaking. I told someone on here a few weeks ago that I've never used 'gonna' and 'gotta' and never intend to. It was only because I wrote that, that, when in conversation, I became aware I had just used one of those words after I said it. As an English learner, I suggest you use standard English. When you're fluent enough, you'll use these slang forms without knowing it!

These are reduced forms. I hesitate to call them slang. Most native speakers revert to these reduced forms when in a hurry.

We do use slang more often that we care to admit; though it is not correct grammar. Here are some examples. Gonna (going to): I'm gonna watch a movie tonight. What are you gonna do? Kinda (kind of): I'm kinda tired this morning because it is kinda early. Gotta (got to; have to): I gotta go to work today. Don't want to but I gotta go to make some money to pay bills.

In speaking, these are just reductions and are perfectly fine. They are not slang. In writing these forms are best for one-on-one texting and informal communication between friends in emails or on social networking sites like Facebook. The idea is that you know the people who are reading your words. In all other writing, those forms are wrong. You can't use them in formal emails, business letters, academic writing of any kind, etc. It makes a person look very uneducated if he uses them in those kind of situations. I think these written reductions look really bad when used in forums as well, where you are writing for people you don't know, but people do it all of the time.