Five months ago, I {decided/have decided} to learn how to play the piano.

Is "Five months ago" a definite time?

I have read that when there is a definite time, I should use the simple past tense. For example:

I ate my dinner two hours ago.

When there is no definite time, I should use the present perfect tense. For example:

I haven't seen my best friend for a long time.

  • The sequence Five months ago, I have [done something] is invalid, because that initial adverbial clause implies a completed action in the past, which clashes with the implied "relevance to time of utterance" of the Present Perfect verb form. Jan 15 at 18:57
  • 2
    ...but many native speakers might be happy enough with I've had lunch two hours ago, even though strictly speaking that should be Simple Past I had lunch... (its fine by me). Curiously, I find that construction completely unacceptable if the adverbial element is "fronted", and the contraction "expanded", to give Two hours ago I have had lunch. Jan 15 at 19:03

Well, if you accept the definition the dictionaries give of AGO, like for example Merriam-Webster:

earlier than the present time (that is, earlier than NOW, the time of speech),

then your expression of time five months ago is very definite.

So your sentence should be:

Five months ago (which is in August 2020), I decided to learn (how) to play the piano. (Gngram prefers learn to play the piano rather than learn how to play the piano.)

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