Grammar

1) If I were you, I would go there by train.

( это 2 тип условных предложений, где после If используют Past Simple or Past Continuous и обязательно глагол to be в форме were, если речь идет о главном глаголе в значении быть....не забывайте ставить запятые, разделяя тем самым условие и следствие, в случае если условие стоит в середине предложения- запятые перед if не ставятся)

2) He would have bought a piano last year but he didn't have money at the time.

( это смешанный тип условных предложений, в котором присутствуют элементы 3 типа; условные предложения 3 типа говорят о ситуации в прошлом, на которую повлиять или изменить уже нельзя; напомню формулу If+ Past Perfect/ Past perfect Continuous, :)+ would have V3/ed  ;

смоделируем 3 тип условных предложений используя пример 

He would have bought a piano last year if he had had money.)

3) We would send a parcel now but the post office is already closed.

( смешанный тип с элементами 2 типа условных предложений)

4) They would go to the cinema in the/this evening but they need to prepare for an/the exam.

( смешанный тип с элементами 2 типа условных предложений, не забывайте про артикли, в данном предложении-  an/the exam, у вас есть варианты, все зависит от контекста, например, если вы не знаете какой экзамен, тогда - неопределенный артикль an, но если вы знаете, что это к примеру экзамен по физике и ранее в тексте о нем шла речь - the; оставить exam совершенно без артикля нельзя, артикль- это своего рода одежда для существительного, так что не оставляйте их голыми:) 

5) I am sure that my brother could help me to repair the flat.

( смешанный тип с элементами 2 типа условных предложений)

6) If not for you, we would lose.

( смешанный тип с элементами 2 типа условных предложений)

7) If it wasn't raining, the children would go to the river.

( речь наверняка идет о конкретных детях)

П.С. рада, если помогла Вам!

Grammar

1) If I were you I would go there by train.
2) He would have bought buy a piano last year but he didn't have the/any money thenat that time.
3) We would have sent a parcel by now but had not the post office being is already closed already.
4) They would have gone to the cinema today this evening but if not for the they need to prepare for their exam.
5) I am sure that my brother could help me to repair the flat.
6) If it is not for you, we would have lostloose. Lose and loose are two different things.
7) If it wasn't raining, the children would have gone to the river.

Grammar

1) If I were you I'd go (there) by train.
2) He would've bought a piano last year but he didn't have enough money at the time.
3) We would send a parcel now but the post office is already closed.
4) They would go to the cinema this evening but they need to prepare for an exam.
5) I am sure that my brother could help me repair the flat.
6) If not for you, we would lose.   (If it weren't for you, we would have lost) 
7) If it wasn't raining, children would go to the river.

Number 7) is correct but are you talking about children in general?

If it wasn't raining, there would be children at the river.

If it wasn't raining, the children would go to the river.

If it wasn't raining, children would be at the river.

:)

Grammar

1) If I were you, I would go there by train.
2) He would have bought buy a piano last year but he didn't have money at the time.
3) We would send a parcel now, but the post office is already closed.
4) They would go to the cinema today this evening, but they need to prepare for an exam.
5) I am sure that my brother could help me to repair the flat.
6) If not for you, we would lose.
7) If it wasn't raining, the children would go to the river.