In this construction with once, can I use present simple or present perfect? Is there any difference?

Once you have bought the car, send me the invoice.
Once you buy the car, send me the invoice.
Once suggests completion, so you should probably use the tense that matches this best: present perfect.

If you wanted to use present simple, you might think about using "when":

"When you buy the car, send me the invoice"

This does cause a difference in meaning: the second suggests immediacy, whereas the first emphasises that the act of buying the car should be totally completed before sending the invoice.

As a better example:

"When you arrive in town, give me a call" - Call me as soon as you arrive
"Once you've arrived in town, give me a call" - Call me after you arrive- need not be immediately.
Hello Smith

I am not kind of stupid to believe Google's majority goes with grammar, but are you sure we always have to use present perfect tense in a "once" clause?

Google results
Once you arrive in town ...308 hits.
Once you've/you have arrived in town...18 hits.

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Well, people will understand what you are saying either way, so no, you don't *** to use present perfect, but I think it is a more accurate and natural usage. The prevalence of present simple for this construct in google probably has a lot to do with the dominance of American English on the web. American English tends to use the present perfect much less than Standard English, ie: "Did you finish your homework?" instead of "Have you finished your homework?".

So, no, in this case I don't see it as a hard and fast rule but I think it's definitely preferable, depending of course on context.
If the serach is confined into AC.UK domain,
Once you arrive in :143 pages
Once you have arrived in :143 pages

So you may be right that Americans prefer simple present tense.

I'm only one, but I prefer the simple present tense. The past perfect seems unnecessarily wordy.

The only difference I detect between "when" and "once" and "as soon as" is the increasing order of immediacy or insistence. Otherwise, all three take present or present perfect with a main clause in the present point of view or past or past perfect with a main clause in the past point of view.
In no case are "will (have)" or "would (have)" felicitous choices after these subordinating conjunctions of time.

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Hello all

1. Once you arrive in town...
2. Once you've arrived in town...

The present tense seems to lend itself to a sense that 'arriving' is near the beginning of a series of actions:

3. Once you arrive, go straight to the High St. I'll be waiting outside Boots.

Whereas the present perfect seems to lend itself to a sense that 'arriving' is near the end of a series of actions:

4. Once you've arrived, give me a call.

But I may be imagining it.

Mr. Pendantic,

Re: your imaginings

I don't think it's imagination.

With the present perfect, but not with the simple present, in the subordinate clause, you can easily use another present perfect in the main clause.

Once you've seen one slum, you've seen them all.
Once you've answered the tenth question, you've finished the exam.

These focus on the completion of some past experience. I don't think this focus is possible with the present tense in both clauses:

Once you see one slum, you see them all. (?)
Once you answer the tenth question , you finish the exam. (?)


Once you see the post office, you are half-way there. (Incompletion implied.)

On the other hand, maybe I'm imagining things too?

That makes sense to me!

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