Can a sentence mix with present tense and past tense? [closed]

I think the presentation is good so I shared it with you.

Is the sentence grammatically correct? I am confused whether a sentence can mix with present tense and past tense or not.

Yes, this sentence is acceptable, but you need a comma after the word good. The two clauses of the sentence are capable of functioning separately as complete sentences:

I think the presentation is good. I shared it with you.

This quality makes your original statement a compound sentence, which requires a comma before the conjunction “so” to make it grammatically correct:

I think the presentation is good, so I shared it with you.

This makes the sentence correct in the technical sense. However, if it sounds awkward to your own ears, you may consider revising it for the benefit of the reader to either separate the clauses or stay in the past tense for clarity. Just because a sentence is grammatically correct doesn’t mean there’s not a more ideal sentence for the situation that you could choose instead.