crescendo/decrescendo question

How do i know when to stop crescendo/decrescendo after finding this expression on a score?

most of the time it'll end at the next indicated dynamic (assuming you mean the word "cresc." and not the < lines)

Yup, or if it is very obvious in the score.

What's the dynamic expression for "now play normally"? hehehehe :)

Dynamic markings are all relative, so there's your answer.

Isn't there a symbol that you can use? I believe you can use a dashed line with a down marking to indicate how long anything of that nature should be (cresc, rit, etc...).

Ex., ---------| (above the staff) or something like that.

it really depends on how long the cresc/dimin is.

If it is one bar long only, usually you put the < or > signs. If it is longer you put cresc. and the doted line (under the staff actually, not above). If it is a really long crescsento, for example 4 pages long, you usually go the logical way and put "cresc. sempre" which means keep incresing volume continuously...

In all cases it is recomended to put dynamics before and after the transition. so p < f or something. If it is really small (say half a bar) then usually you don't go the extra effort, especially if you have in mind to go from p to mp. If you go in a short period of time from p to fff then it would be recomendable to put the additional instruction of "molto" above the < sign.