have + past participle, versus did + verb?

Question

Here are some pairs of sentences. I don't know the difference in meaning between each sentence in each pair.

How many apples did you eat?
How many apples have you eaten?

How many bags did you use?
How many bags have you used?

Questions:

1. What is the difference in meaning between the two constructions?

2. Grammatically, what is the structure of "How many bags have you used"?

For example, for "how many bags have you used?", I notice that "used" is a past participle, but I'm still not sure how I might construct this sentence. I suspect my knowledge of grammar is lacking.

Here is one attempt:

• "[You] [used] [five bags]" -- simple subject-verb-object sentence that uses very simple grammar that most beginners learn.

• "How many bags did you use?" -- constructing a question, that can be answered with the previous sentence, by using "how many" at the beginning and doing an inversion of the [subject + verb] and [object]

• "How many bags have you used?" -- ???? No idea how to construct this sentence! Does it have something to do with the passive voice? Where did the word "have" come from?

Answer 1

Have/has + past participle = present perfect, which is the tense of the 2nd and 4th sentences. Simple as that. Just follow the above structure. Your 1st and 3rd sentences' tense, however, are simple past: Did + infinitive verb without "to". As for the difference in meaning between the two structures, present perfect has several applications in the English language. It usually denotes that an action started at an unspecified time in the past, and it is still relevant. So, the asker does not need to know when you started eating the apples - the specific ones and not apples in general - just how many you ate right until she asked the question. The 1st and 3rd questions are talking about a specific time in the past. Obviously, this is implied and understood by both the asker and the hearer - that is probably why it is not mentioned - like this morning, or yesterday when you got sick, for instance. It started in the past and finished in the past so, it is an action that was completed in the past. In the present perfect tense, the action may or may not have stopped. You may still continue eating more apples, but we don't know. Also, not all present perfect tenses have the same time reference. Ex: how many apples have you eaten today vs. how many times have you been to Japan? None of the above sentences are passive.