Does the sentences mean I still have the computer?
I've bought a computer since last year.
I've bought a computer this year.
I've bought a computer this month.
Does the sentences mean I still have the computer?
I've bought a computer since last year.
I've bought a computer this year.
I've bought a computer this month.
The OP's sentence “I've bought a computer since last year” is not idiomatic. The Present Perfect and the Present Perfect Continuous are often used with since when it indicates either a specific point in the past at which something happened and did not continue, or a point in the past that signals when an event started.
since (preposition)
in the time after (a specified time or event in the past): from (a point in the past) until the present time
- I haven't seen him since yesterday.
- I haven't eaten since breakfast.
- Since the party, she has not spoken to him at all.
- We've been waiting for you since 10 o'clock.
- The company started as a small local business 10 years ago and has grown a lot since then. [=within that time]
Source Merriam-Webster Learner's Dictionary
I've bought a computer this week/month/year.
This denotes a SINGLE action. The purchase was completed at a certain point in the past. The person is not currently buying that same computer a day, a week or several years later. Moreover, the time expression “this week/month/year” refers to a period of time that is not yet concluded. Consequently, the reader will surmise that the buyer now possesses the product. In fact, the present perfect is used for actions that were completed in the past but have an effect in the present, e.g. She's cut her finger. (Now her finger is bleeding.), We've finished paying off the mortgage (Now we no longer have to pay back the bank.), The government has raised income taxes for the second time (Now workers have less spending money.)
However, when consumers cannot afford to purchase an item, they may decide to pay monthly installments. In which case, the present perfect continuous is often used for actions repeated over a period of time.
I've been paying for my car in installments since last year.