This is probably a very stupid question; What is the difference between « j’ai » and je l'ai »? What is the l' part there for? Can you give me sentence examples of them being used?

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J'ai = I have
Je l'ai = I have it

In fact, "l'" is used before a vowel instead of "le" or "la". Le / la is used to replace a noun.
For example :
"J'ai le livre.", But we already know what we're talking about, so we don't want to say "le livre". So we say :
- Je le ai.
But it doesn't sound well because "ai" starts with a vowel, so we say :
- Je l'ai.
We never use le / la in front of vowels, always "l'"

Le is for masculine nouns, and la is for feminine nouns. So :
- J'ai la pomme. => Je la ai. => Je l'ai.

Before a consonant, we use le / la :
- Je fais le devoir. => Je le fais.
- Je fais la cuisine. => Je la fais.

I hope it helps :)