Teach me about contradictions

[Ben Davis]

I understand that in the English language you can say things like "I do not feed cats until Sunday."
Or "I do not feed cats if it is not a Sunday"
And if the quran included the words "Until" or "If" in the verse this would mean no contradiction.

But to have a verse that specifically says "God does not forgive shirk" and he doesn't give an exception to this rule even in the same verse.

What you say about the English Conditional is true however that's not the only mechanism you need to consider, especially when we're talking about such a contextual language as Arabic (or consequent translations) in such a poetic setting as the qur'an. 'Does not' is not necessarily an absolute (unlike 'never') and the source Arabic may be one of many different negations, each with its own context and poetic usage (I don't know which negation is used in the verse you're referencing) therefore it may not be an absolute either.

In fact I don't even think this is a contradiction within the quran but a contradiction on what the quran says and what Muslims believe in order to increase the number of christian converts.

Entirely plausible: many religions have co-opted local customs in order to propagate themselves and their culture spread. Think of Christianity and its absorption of Celtic, Pagan and Norse traditions when introduced to Europe.

But there are some hadith verses which contradict god not forgiving shirk and some quran verses which say god forgives all the sins of his slaves, whoever his slaves are? I don't know if he considers christians to be his slaves aswell.

If there are many seeming contradictions, I think it's likely that the verse is loose enough to allow them; it's generally when we see only one or 2 that they're most likely to be contradictions.