The difference between ‘have’ and ‘has’

Between ‘have’ and ‘has’, ‘have’ is the main verb which means ‘possess’.
e.g. They have a lot of things to do today.

On the other hand, ‘has’ is the singular form of ‘have’.
e.g. She has a cute doll.

‘Have’ is also use after ‘do’, ‘did’, ‘can’, ‘could’, ‘will’. ‘shall’, ‘would’ and ‘should’.
Examples of the usage of ‘word’ with the above words are:
a) I do have a book like this.
b) He did have a pen like yours last year.
c) You can have my share if you like.
d) I will have my breakfast late today.
e) We shall have to hurry or we will miss the bus.
f) I would have done the same if I were you.
g) You should have taken the earlier train.

Sometimes, ‘have’ and ‘has’ can appear together in a sentence.
e.g. She has to have great courage to accomplish such a feat.