[susan53]

This idea that there are "four" conditional structures is rubbish. What is said in the first clause and what comes in the second are entirely independent of each other - it depends entirely on the meaning that the speaker wants to express. Have a look at these for instance - all perfectly possible sentences, but not matching any of the so-called "four" conditional structures :

If he'll agree to give us the money, all our problems are over!
If I hadn't had the accident, I'd be in the Bahamas now.
If you would stop making so much noise, we'd be able to get some sleep.
If the train was on time, they'll be here soon.
If you study more, you might have a chance of passing.
If you studied more, you might have a chance of passing.


You can only explain the form of each verb by looking at the meaning that the speaker wants to express in each clause, independently of what is in the other.

I know that doesn't answer your question , but it may be a reason for your confusion. Basically the "zero" conditional is any sentence following the pattern :
If clause - present simple, main clause present simple. So eg : If you press the button, the light comes on
while a "first" conditional is any sentence following the pattern :
if clause - present simple, main clause - will+ infinitive. So eg : If she doesn't apply for a scholarship, of course she won't get one

If you just want to identify them, that's the traditional description. But don't be surprised if you find a lot of other combinations. There's nothing "special" about just those two. Your own examples show this. In (5) for instance you have If + present simple , main clause - imperative; and in (6) if + present simple , main clause should + infinitive. These are neither zero nor first conditionals - nor second nor third. That's why you can't identify them.

There is no grammatical rule that says " If if is followed by XXX then the main clause must contain YYYY". Any verb can occur in either clause as long as the meaning is logical. The only one above where the meaning isn't logical is *If she doesn't apply for a scholarship she doesn't get one. This makes no sense because you have used a verb that expresses a permanent fact whereas you need to express a definite prediction : she won't get one. But it's the meaning that determines this, not some abstract grammatical rule.