Except for the second couple of sentences, all the other sentences have something in common: They negate a verb.

If you are negating the verb in a question:

- If the verb is _be_, you replace _are_ with _aren't_, and _is_ with _isn't_
- If the verb is another one, you generally replace _do_/<em>does</em> (the auxiliary verb) with _don't_/<em>doesn't</em>

If you are negating the verb in a sentence that isn't a question:

- If the verb is _be_, you replace _are_ with _aren't_, and _is_ with _isn't_
- If the verb is another one, you generally add _do_/<em>does</em> (_does_ is for the third singular person) before the verb

In your examples, _practice_, _come_, _marry_, _care_, _realize_ are all verbs, and they are not _be_.

"What is it?" is correct if for example you are asking what the object you are looking is. "What does it?" would eventually be "What does it do?" and it is asking what the purpose of an object is.  
When you ask "What is it?" people normally say what that object is; if they think you didn't understand what it is, they could also explain what the purpose of that object is. Although, the two questions have different meanings. 

Notice that _matter_ is also a noun; as such, it is used in sentences like the following ones:

> It is a matter for the police.

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> It is not a matter of not being able to do something; it is just a matter of knowing the right people.