For example: How long will you have been studying** when you graduate?**

Will you have been studying - future perfect continuous

graduate - simple present.

The same sentence contains two different verb forms. I thought the same verb tense was carried throughout the sentence.

  • 1
    There is no rule that says you cannot use different tenses in a sentence. Really, there is not. – oerkelens Dec 8 '17 at 12:05
  • When the first clause is in the future or present, the verb after when is present. – Lambie Dec 8 '17 at 13:41
  • Yeah, "when you graduate" just means at the exact moment that you graduate in the future, you "will have been studying" how long from now to then. – Nick Dec 8 '17 at 15:04
  • “How long will you have been studying by the time you graduate?” sounds much more natural to me. – mamster Dec 8 '17 at 16:05

Whether or not there is a canonical sequence of tenses in English is debatable. What is not debatable is that the structure

"If[when] time indicated by conditional verb, then future time relative to conditional verb" is idiomatic.

"If I get home early, we will go out to dinner."

"If I got home early, we would go out to dinner."

"If I had got home early, we would have gone out to dinner."

The idea that all the tenses in a sentence must refer to the same temporal frame is a gross over-simplification of English grammar.

Look at this sentence. It contains an independent clause and dependent clause:

When I get home (dependent clause), I will call you (independent clause).

Many dependent clauses begin with words such as if, when, before, after, as soon as, and until. When a dependent clause talks about a future time, use the simple present in the dependent clause and the future with will and be going to in the independent clause. Both verbs are future in meaning. The simple present shows the future action, and the future shows the second future action. For example:

When we get to Italy, we'll rent a car.
I'll contact you as soon as I hear from her.

Different verb forms could go in a given sentence and the sentence still be valid. You simply need to know exactly what you want to say and the variation in meaning between the different forms of the verb(s) you're using.

In your sentence, and since you're taking about a future scenario, it's only natural that you will be using future tenses. The perfect continuous is to ask about the accumulation/completion of some relatively previously started activity/state, studying in your sentence. The conjunction when introduces the point up to which that activity is considered/measured.

For example, you could've also said:

-How long will you have studied when you graduate?

-How long will you have studied when you have graduated?

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