I agree with Timothy and Jasper. The first is correct. The second should be: "I asked him: 'what is the biggest mistake parents make?'" I'd say 'what' doesn't need a capital letter, because it's still part of the same sentence. The third sentence is correct.

You can use a colon to introduce an item (or items), or to explain the first part of the sentence. There are two parts to this sentence: 1. Who would you rather spend your evening with, and 2. me, or a pickled toad? 1. is an independent clause, because it's a complete sentence on its own. 2. is a dependent clause, because it's not a complete sentence on its own. Yes, you can use a comma between an independent clause and a dependent clause. You can never use a comma between two independent clauses. This is called a 'comma splice.' However, I think a colon is better here, because it makes the sentence clearer and easier to read. Who would you rather spend your evening with: me or a pickled toad?

Hello, Mostafa. When we say 'what did you do?' we are talking about one (specific) time in the past. We are asking about one action, which started and finished in the past. We're using the present simple tense. When we say 'what have you done?' we are talking about a time in the past - but it's not specific. We are asking about either one action that you did, but the specific time isn't important, or we're asking about more than one action in the past. We are asking for more of an explanation. We want to know more information. We're using the present perfect tense. A: 'What did you do yesterday evening?' B: 'I went out for dinner and watched a movie.' [ NOT 'I have went out for dinner and a movie'] A: 'What have you done today?' [no specific time - any time today] B: 'I've done my homework, been for a walk, and washed the dishes.' ['I did my homework, went for a walk, and washed the dishes' is also okay.] There is also a difference in tone: 'What did you do?' can sound concerned or irritated. A: 'What did you do?' [concern - same as 'what happened?'] B: 'I fell off my bike, and cut my leg.' A: 'What did you do? [irritation] B: 'I was drinking coffee, and accidentally knocked it onto your homework.' 'What have you done?' can sound angry. A: 'What have you done?' B: 'Sorry, I accidentally knocked my coffee onto your homework.' A: 'That's due tomorrow! I've spent hours writing that!'