I've bought a car since last year; I've learnt to swim since 2017

Question

Are the two sentences correct?

1. I've learnt to swim since 2017.

2. I've bought a car since last year.

Context for 1:

I moved to this city in 2017. 2017 was a turning point in my life. Before 2017, my life was a mess. Since 2017, my life has changed a lot. I've married a pretty woman. I've found a decent work. I've made lots of friends. I've also learnt to swim since 2017. Before 2017, I tried several times to learn it, but eventually gave up because I was in a bad mood.

Context for 2:

A: Did we meet last year?

B: Yes, we did.

A: Are you still riding your motorcycle to work?

B: No. I've bought a car since last year.

Answer 1

What they mean is:

"Some (unspecified) time after the beginning of 2017 I learned to swim."

"Some (unspecified) time after the start of last year, I bought a car."

• A: "Did we meet last year?"

• B: "Yes we did."

• A: "Have we met since last year?"

• B: "Yes we did, we met last month, don't you remember?"

On the other hand, this sounds unnatural:

• A: Are you still riding your motorcycle to work?

• B: No. I've bought a car since last year.

I would write it as "No. Since last year, I bought a car."

In this context, though, the usage suggests that there was a specific incident that prompted B. to have bought a car, for example: "Since (that time when I had that crash) last year, I bought a car.

Note it should be "I bought" not "I've bought". I find it difficult to explain why.

Answer 2

I moved to this city in 2017. And 2017 was the turning point in my life, my life has changed a lot since then;

Here the Present Perfect is used correctly because the speaker's life is ongoing, other new and positive changes might occur further on in his or her life. The present perfect is also used to describe a state or event which began in the past and continues to the present time.

I got married, found a decent job, made lots of friends, bought a new car but I also learnt how to swim. Before 2017, I tried learning it several times but I always gave up because I felt unsatisfied.

Here I would use the Simple Past because the speaker knows when they got married, found a better job, made new friends etc. The time of these events is fixed in the speaker's mind. Although the reader knows the marriage must have taken place sometime between 2017 and 2020, they might infer that it happened before 2020 seeing as we are in the month of June and the COVID pandemic exploded between February and March of this year. It's unlikely that any of the above events happened after January 2020.

According to Advanced Grammar in Use by Martin Hewings, first published in 1999, the adverb “since” is normally used in the present perfect but it can also be used with the simple past if it is embedded in a clause and the main clause is in the Present Perfect.

In a sentence which includes a since-clause , the usual pattern is for the since-clause to contain a past simple, and the main clause to contain a present perfect:

• Since Mr Hassan became president, both taxes and unemployment have increased.

• I haven't been able to play tennis since I broke my arm.

However, we can use a present perfect in the since-clause if the two situations described in the main and since-clause extend until the present:

• Since I've lived here, I haven't seen my neighbours.

The teacher of English writing for the BBC site, Learning English, elucidates

The tense in the since-clause can be past or perfect, depending on whether it refers to a point in the past or to a period of time leading up to the present or, in the case of the past perfect, leading up to a point in the past. […] Note also in these examples that present and past tenses are possible in the main clause as well as the present perfect:

• Henry’s been teetotal since we got married.
  Henry’s been teetotal since we’ve been married.

• It’s only a week since I met him, but we’re very much in love.
  It’s only a week since we’ve known each other, but we’re very much in love.

• They’re a lot happier since they separated.
  They’re a lot happier since they’ve been living apart.

• You’re looking much better since you came out of hospital.
  You’re looking much better since you’ve been out of hospital.

Answer 3

Both your examples are unusual. Neither is idiomatic. That's to say that you are not likely to hear native English speakers using them.

While you might well hear I've have been swimming regularly since 2017, you are unlikely to hear I have learned to swim since 2017.

Far more likely is either the omission of since 2017 or use of the simple past as in I learned to swim a few years ago.

Alternatively, I have been learning to swim since 2017 (grammatical but unlikely).

The same applies to buying the car. The likely options are:

No, I've have bought a car.
No, I've since bought a car.
No, I've bought a car since then.
No, I bought a car (a few months ago)
No, I've been driving a car since the end of last year.

Since is more likely to be followed by a continuing activity than a single action - with lots of exceptions.

There are numerous guides online to the use of since. Google use of since and/or see: https://dictionary.cambridge.org/grammar/british-grammar/since
https://www.macmillandictionary.com/dictionary/british/since
https://sentence.yourdictionary.com/since