Internal server error is 500 error, so to fix it you need to check logs and get to the root of error. It could be anything like not properly defined function, timeout. There need to check logs.
Syntax error is improper coding error like missing semi colon, comma other syntax misspelling. So to fix them code fix is required.
Database connection error can be resolved by correcting the entries in wp-config for database like user name, db name, password, host and allowing proper permission to db user.
We can discuss about project if you are interested.