First Conditional in English

In this type of conditional sentences we have Present Tense in if-clause and will or imperative in the main clause.

In this type what is said in the main clause id dependent on something which may not happen though this 'something' is assumed by the speaker.

If you park your car there, lock it and leave the key here.

Park your car on the left if you can find the space.

If you should be interested in it I will send you my copy of the book.

Should you (by any chance) be interested in it I will send you my copy of the book.

This idea of dependence on chance or on somebody's else wishes is usually emphasized by stressing 'should'.

Drink your soup or I will smack you. = If you don't drink your soup, I will smack you.

Drink your soup and I will smack you. = If you drink your soup, I will smack you.

Let go of her or I will shoot you.

Let go of my hand.

If you don't let go of her, I will shoot you.

This type of conditional sentences assumes an open condition which may or may not be fulfilled. If the condition represented by the conditional clause is fulfilled, the state (action) represented by the main clause will follow.
If you tease the cat, it will scratch you.
If you tease the cat, it can scratch you.
If you tease the cat, be prepared it will / can scratch you.

In the first type, the time reference may be either present or future. This may be understood:
If you tease the cat (in future), it will scratch you (in future). This sentence refers to one particular event.

We may also have conditional referring to general truths or habitual behaviour, where instead of will + infinitive the present tense can be used in the main clause as well, e.g.
If you tease the cat, it may scratch you.
Such a statement does not refer to a particular situation but to a general behaviour. Statements of this kind often appear in scientific writing, e.g.
If we boil water, it evaporates.

Use

The First Conditional is used for the situations which will take place in the future.

Form

	'If' -clause: present tenses	main clause: 'shall/will' future
be:	If I am better tomorrow,	I will go to work.
have:	If I have a headache,	I will take an aspirin.
simple present:	If he finishes early,	he will go home.
present progressive:	If you are standing in the rain,	you will catch cold.
present perfect:	If he has arrived at the station,	he will be here soon.
present perfect progressive:	If he has been travelling all night,	he will need a rest.
can, must:	If I can afford a car,	I will buy one.

Variation I: 'If' + present + modal

	'If' -clause: present tenses	main clause: modal
simple present:	If she finishes early,	she can/could phone me. she may/might phone me. she should/ought to phone me. she must phone me.
present progressive:	If she is arriving today,
present perfect:	If she has arrived,
present perfect progressive:	If she has been waiting,
can, must	If she can't understand it,

Variation II: 'If' + should + e.g. imperative

'If' -clause: present tenses	main clause: e.g. imperative
If you (should) see him,	please give him my regards.
Should you see him,
If you (should) happen to see him,
Should you happen to see him,

Variation III: Imperative + conjunction + clause

imperative	conjunction	main clause: 'shall/will'
Provide the materials	and	we'll do the job.
Stop shouting,	or	you'll wake up the neighbours.
Put that down,	or else	I'll hit you.
Be there on time,	otherwise	you'll lose the job.