If you are using a GUI based application like Microsoft Word of Google Docs, it would be easier for you to know the number of word occurences in a file. But what if you are in a terminal? That's where both grep and wc comes handy tools.

Let's say you have the following text:

The European languages are members of the same family. Their separate existence is a myth. For science, music, sport, etc, Europe uses the same vocabulary.

Now, you can use the grep and wc tool to count the number of times "the" appears in the file:

grep -o -i "the" file.txt | wc -l

Hope this helps you out!