A clause is an independent logical fragment of a sentence, which contains an actor and an action. Every sentence contains the following:

• A main clause
• Zero or more subordinate clauses

Subordinate clauses modify the idea in the main clause. As the name implies, subordinate clauses are less important than the main clause. For example, consider the following sentence:

Python is an interpreted programming language, which was invented in 1991.

• Main clause: Python is an interpreted programming language
• Subordinate clause: which was invented in 1991

You can usually identify subordinate clauses by the words that introduce them. The following list (by no means complete) shows common words that introduce subordinate clauses:

• which
• that
• because
• whose
• until
• unless
• since

Some subordinate clauses begin with a comma and some don't. The highlighted subordinate clause in the following sentence, for example, begins with the word because and does not contain a comma:

I prefer to code in C++ because I like strong data typing.