Syntax error highlighting not showing on Java projects

I have Intellij Idea 2017.2.2 X64 student edition

I have only recently started using Intellij after switching from Eclipse.

My problem is that the red squigly lines that is shown under wrong codes and the yellow ones under unused variables for example (I'm not sure if it is even called "error highlighting" or not) does not appear at all when I am working on my assignment Java projects that I got from my lecturers. They do appear normally when I create my own project.

I have tried deleting the .idea folder and clicking File -> Invalidate Caches / Restart and playing around with the inspection setting. I have tried looking around the web for solutions but nothing seems to talk about my problem. 

Also, the auto-complete feature for methods from imported library suddenly stopped working. For example, I imported the Math and arraylist libraries but when I typed "Math." or the name of my arraylist, the dropdown list for the auto-complete does not show for Math. and the list that is shown by my arraylist is unfamiliar to me.

I don't know if any of this matters but the project has an ant build, a bitbucket pipeline .yml file, and Eclipse' .classpath and .project file which I suspect I get because I opened the project using intellij after working on it on Eclipse.