Much much too wide a topic, Guest. Can you narrow the topic, perhaps give a few examples of your problem areas. I can't imagine that someone who writes as well as you have here would have any problems.
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When using the present tense (with 'to have' as the main verb), or the present perfect tense (with 'to have' as the auxiliary verb), 'has' is used only in the third person singular-- that is, when the subject is a single entity that is not you or I: he, she, it, John, the Eiffel Tower, etc.
I have a headache.
You have a headache.
haveis used withI,you,theyhasis used withit,she, andhe.
The countable and uncountable nouns sometimes can create lots of confusion as to whether to use singular (has) or plural (have).
Police, as strange as it might seem,