When I need to learn a programming language quickly, or just recall it, I often use a few tips, very simple.

Data Types

Also known as primitive data types. Data types are the very basis of every programming language. Knowing them allow you to manipulate variables, arrays, functions, etc. Generally, there are less than 10 of them. For example, Java has the following primitive data types:

byte
short
int
long
float
double
char
boolean

String could also be considered as a primitive data type since it doesn't require the keyword new to be instanciated.

Keywords

While we're on the subject, keywords (also called "reserved words") are another excellent way to quickly recall a programming language. Here again, they are not so numerous, generally around 50. Here are some keywords in Java:

abstract
boolean
break
case
class
continue
default
...

Those words are reserved and used by the compiler, that's why you can't use them as variable names for example. Consequently, they give you a pretty clear overview of what you can do with each language. Furthermore, if you've already learnt that language, you'll then quickly remember what each keyword means. Then, just iterate through those keywords and in less than 5 minutes, you'll remember everything about this language!

The main function

Each language has its own approach. Here are some of them:

Java

public static void main(String[] args) {
    ...
}

C

int main(int argc, char *argv[])
{
    ...
    return 0;
}

Python

def main:
    ...
if __name__ == "__main__":
    main()

Don't forget to know this function!

Have a look at some of your previous projects... or code

Giving a quick look at projects helps a lot! Try to look at the basics: inherited classes, overloaded and overriding functions, asbstract classes, IO functions, etc. If you have time, try to code some of them again!

(Re)Learn A Programming Language Quickly