一年半前にって英語でなんて言うの？

The best way to describe one year and six months of time is "a year and a half ago." "Began" is another way to say start. I hope that this helps :)

Well, if your memory is perfect you may be able to recall exactly how long you have been playing tennis as in the first sentence. Otherwise you can round up or down to the nearest year, as in the second example.

Answers 1 & 2 explain to someone that you started playing tennis exactly a year and a half ago.
Answer 3 is a estimation of the time you have been playing tennis for. You may not remember the exact date, but when we use the word 'about' in this context it means roughly or close to a year and a half.

If you started playing tennis one year and six months ago and you would like to explain this, you could say any of the following: "I started playing tennis a year and a half ago", "I began playing tennis a year and a half ago" or "I've been playing tennis for about eighteen months". All three of these sentences adequately explain what you are trying to communicate.

It is most common for people in Commonwealth countries to use 18 months as a quantifiable number. Even when a baby reaches a 1 1/2 you will more readily hear a parent saying 18 months old. The alternate way of saying just 6 months shy of 2 years makes it sound longer to the audience but technically still correct.

To tell somebody when you started playing tennis, you can say:
"I started playing tennis a year and a half ago."