Last week I looked at how to use the word 'because'. One of my students then asked me to explain the difference between 'because' and 'because of'.
Compare these two sentences:
I stayed at home because the weather was bad.
I stayed at home because of the bad weather.
The important difference is the use of the word 'was' in the first sentence. After 'because', you need a subject and a verb (e.g. the weather was). After 'because of', we don't use a verb, only a noun, nouns or noun phrase.