What's the meaning of the zero conditional or the first or the second .. Does it mean the form of the verb and what does that exactly mean does it mean infinitive although the present simple is used! so what does the numbers in the .. conditional mean?
The zero conditional is called that, because it is not really a condition.
When speakers present an action or state in factual conditional terms (the so-called Zero Conditional), they are stating that they accept that action or state as reality
If you heat ice, it melts.
If Andrea cooks, I wash up.
If it’s ten o’clock already, then I’m late.
In , the melting of the ice is presented as a consequence of heating it - on every occasion that it is heated. This is a consequence independent of time; it is a general truth based on physical law and this construction is frequently used in scientific writing. In practice, the meaning is similar to utterances using when or whenever:
1a. When[ever] you heat ice, it melts.
There is a slight difference in meaning. [1a] implies that ‘you’ do heat ice on occasions;  allows that you may not actually do it (although states that the consequence of a present or future heating of ice is the same as all past heatings by anyone - it melts).
In factual conditionals that are general truths, we normally use the Present Simple in both clauses; it is the natural tense for unmarked time. It is possible however to use modals; will, for example, adds the idea of absolute certainty. Note that in [1b] we are presenting a general truth, not a future possibility.
1b. If you heat ice, it will melt.
Other tenses and aspects of the verb, as well as modals, are also possible in either clause in the appropriate situation:
- If water has been boiled for twenty minutes, it is completely sterile.
- If the metal snaps, it has been subjected to extreme stress.
If a dog is wagging its tail, it’s happy
If you can speak Swedish, you can understand Danish.
In , If Andrea cooks, I wash up, the speaker’s washing up and the consequence of Andrea’s cooking, are presented as habitual acts. As with general truths, the meaning is similar to utterances using when or whenever. However, unlike general truths, which are not limited in time, habitual acts can be limited in time, and a wide range of tenses and modals is possible, for example:
- If Andrea cooked, I washed up.
- If Andrea cooked, I would wash up.
- If Carol is working in the garden, Peter often goes down to the pub.
The setting of the situation in  in the past can result in a sentence that has the same form as a hypothetical conditional (referring to future time) or counterfactual conditional (referring to the general present):
- If Andrea cooked, I would wash up. But she doesn’t, so I won’t.
- 0 conditional
If it rains, I take my umbrella.
"If it rains, I'll take my umbrellla.
If it rained, I'd take my umbrella.
If it had rained, I would have taken my umbrella.
In the 0 conditional "if" can usually be interchanged with "when".