To go ahead means to start/begin To go on means to continue Of course there are other meanings to these phrases by themselves but this answer is for if you're comparing the two phrases.

When I was a kid, my mother always took me to the park and I played with other children Other: My mother always took me to the park when I was a kid, where I played with other children.

All of these sentences are technically correct. Classes and courses (in this case) are interchangeable for 1 and 2; they both mean the same thing. But, if you want it to sound more fluent, you "take courses" and "go to/went to classes" Example: 1) I took English courses before 2) I went to English classes before 3) I took English courses for years

How much of this did you buy? The second sentence is ok.