Why is it better to write "I realised I had left" than "I realised I left"?

It seems obvious that the realisation was after the forgetting: to generalize, if you realise something you must have done it before, past perfect should be a bit redundant in most cases. So past simple should be better or it is not the case. I don't understand the logic here.

  • Why would you think it's "better" to use Simple Past after I realised? Per this NGram, most writers obviously don't think that (plenty of written instances of Past Perfect I realised I had left, but none at all of Simple Past I realised I left). Dec 18, 2021 at 16:59
  • Yes I know but my question is why. I tried to explain why I think that past perfect should be redundant because when you realise something it is obviously because something has been done before.
    – Yves Lefol
    Dec 18, 2021 at 17:12
  • oic. Yes - logically, it's "unnecessary" to use Past Perfect after I realised [something which must have occurred before I realised it]. But as that NGram shows, we nearly always do use it in such contexts. But much depends on the specific verb - if we make the tiny change from left to felt, suddenly everything changes. Dec 18, 2021 at 18:25
  • That's because when we realise something about our feelings, most likely we're becoming aware of current feelings. (It would be a bit weird to only realise later how you had felt previously! :) Dec 18, 2021 at 18:28

1 Answer 1


Based on this ngram, it's about twice as common to say "I realized I had left (the keys in the car)" than "I realized I left (the keys in the car)".

The leaving occurs before the realization. Therefore, it makes sense to use past perfect, which expresses the fact that something occurred earlier in the story.

Another possibility when telling a story set in the past, is for something to be "still happening", "continuing to happen". In other words "in the present tense" of the past story.


"I realized I was right!"
"I realized I was in trouble."
"I realized I was hitting the button for absolutely no reason, because the elevator was out of order."

Notice how past perfect isn't used here.

In these cases, the verbs are in the simple past, and indicates a similar time frame, in the same past.

If you convert them all to the present tense, the verbs shift to present tense.

What is the difference between these cases and the original problematic sentence with "left"?

It seems, that we express "left" in the past tense, as something that occurred earlier.

And, it did occur earlier, factually. One thing happened before another thing.

That is not always the case, as shown by the above examples, but it is the case with "left", and countless other actions that may have occurred before the current point in time.

The argument this is understood from context, and redundant, is not valid. Grammar doesn't work that way. You can't discard articles ("the", "a") or plurals ("s") because they are probably obvious, and therefore redundant. You shouldn't discard simple past tense, or future tense, and only use the present tense everywhere, because often it's probably obvious, and therefore redundant.

Actually, we can imagine languages that don't have "past perfect". But, since the verb tense exists in English, and applies to certain situations, then it's often preferable to use it.

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