1. trousers без "a", потому что это в множественном числе
last night, I bought a shirt and trousers online
2.my best friend is very good at sailing
или my best friend is a very good sailor
3.i really like buying clothes online
4.when we were younger we used to visit my grandparents every weekend
ответ: 1 last night, I bought a shirt and a trousers online. 2 my best friend is very good at sailing. 3 I really like buying clothes online. 4 when we were younger, we use to visit my grandparents every weekend.
больше ошибок я правда не вижу