DEAR ALL C HINGS......
who can tell me the different between F-5 and F-20....thanks~

Basically the F-20 Tigershark is another version of F-5, originally called F-5G.  It has upgraded radar and engine power, but that's all about it.  I don't remeber anyone using F-20 except Taiwan.
The whole F-5 range was a light weight short range low level fighter design.  The entire range was dropped in early 1980's due to the F-16, which is far more advanced.
The F-5 performance is comparible to MIG-21 or the chinese F-7 (still in service!!!).  So you can see in Top Gun movie they were using the F-5 as MIG simulations.

thanks a lot from your info....^^
I know that one of them use twin engines.....

thanks ^^
I got it....