Answers

2014-11-24T17:36:00+08:00
The - S form is very commonly used for main verbs and auxiliary
verbs in the 
simple present tense when the subject is or means
he, she, or it. The - S form is also very commonly used as a full
(has) or contracted ('s) auxiliary verb in the 
present perfect tense
when the subject is or means he, she, or it.
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2014-11-24T17:36:56+08:00
We use that when the verb is singular
when the verb is in base form

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