I cannot import things (Jar missing?)

Hi, I am just starting to learn how to make mods. I have set up Gradle and opened eclipse in the right directory (Forge\eclipse). When I first started it up it gave me an error, being a regular Eclipse user I proceeded to remove one of the Jars that was creating an error. This turned out real bad, everything un-imported and I forgot the Jar name... (I have a feeling it started with "undefined"). If anyone can help me I'd be very grateful. 

"being a regular Eclipse user I proceeded to remove one of the Jars that was creating an error"

How is that a 'standard solution'... No...

Re-run the eclipse task and reload eclipse.

So, one thing, if you were a regular IntelliJ user, would you know too remove a Jar when there is a red ! on the Project? I wouldn't think so...

Anyway, I do not quite understand what you are saying, could you re-phrase?

Aside from that, I managed to re-install forge, here are the Jars that are imported; http://prntscr.com/5ms74m

I get an error when trying to re-install it;

:makeStart FAILED

FAILURE: Build failed with an exception.

* What went wrong:

Execution failed for task ':makeStart'.

> Cannot find System Java Compiler. Ensure that you have installed a JDK (not ju

st a JRE) and configured your JAVA_HOME system variable to point to the accordin

g directory.

* Try:

Run with --stacktrace option to get the stack trace. Run with --info or --debug

option to get more log output.

BUILD FAILED

Total time: 2 mins 48.829 secs

I watched multiple tutorials, along with Oracles tutorial, it seems it didn't work, can anyone help me? (: