It is usually incorrect to use a future form in the if clause:

x  If the weather will be good tomorrow, we'll have a picnic.

If the weather is good tomorrow, we’ll have a picnic.

Use

We use the first conditional to describe possible future events or situations and their results:

If the result of the test is negative, you’ll receive notification through the post.

The bank will be starting a recruitment drive if it receives head office approval.

SECOND CONDITIONAL(This is also known as the unlikely or improbable conditional)

Patterns

if + Past Simple would/ wouldn’t + Infinitive

If they wanted to make an offer, she would listen and think it over.

We can also use the Past Continuous or was going to in the if clause:

If a celebrity were staying in the hotel, security arrangements would be tightened.

We often use a modal in the main clause, especially might or could:

More funding for AIDS research could be secured if people were more aware of the dire situation in Africa.

! With be in the if clause, we usually use the subjunctive were for all persons. You may hear English speakers using was as in the first example above. It is becoming accepted today, but you should not use it in formal situations.

[If the programmer was familiar with this language, it would be an easy job.]

If the programmer were familiar with this language, it would be an easy job.

! It is possible to use would in both clauses in US English but not in British English:

US:      The blockades wouldn’t happen if the police would be firmer with the strikers.

Br:        The blockades wouldn’t happen if the police were firmer with the strikers.

 

Use

The second conditional describes an improbable future event or situation. The condition is unlikely to be fulfilled because the future event is unlikely to happen:

If the result of the test were positive, we would call you within two days.

THIRD CONDITIONAL (This is also known as the past or impossible conditional)

Patterns

if + Past Perfect (Past Perfect Continuous) would + Present Perfect

If someone had been teasing your child so nastily, you would have behaved in the same way.

We can use a modal in the main clause, usually might or could:

It might have been easier to break the news if I had known her a bit better.

If the spy had intercepted the message, he could have averted the crisis.

! In US English, it is possible to use would have in the if clause. This is becoming more common in British English, although many people consider it incorrect:

US:    If the play would have finished on time, we wouldn’t have missed the train.

Br:        If the play had finished on time, we wouldn’t have missed the train.

Use

The third conditional describes a hypothetical situation or event in the past. The past situation or event is contrary to known facts, i.e. it is an unreal or impossible situation:

I would have used your builder if I had managed to contact him. (but I didn’t manage to contact him)

MIXED CONDITIONALS

Mixed second/third conditional

Mixed conditionals include the verb forms from two different conditional patterns. These are the two most common mixed patterns.

This conditional describes a hypothetical situation or event in the present, which is contrary to known facts. The result in the main clause refers to the past:

If the island were still a tourist attraction, last week's earthquate would have caused far more deaths. (= The island is no longer a tourist attraction so the earthquake didn't cause a huge number of deaths.)

Mixed third/second conditional

This conditional describes a hypothetical situation or event in the past, which is contrary to known facts. The result in the main clause refers to the present:

If Fleming hadn’t discovered penicillin, there would be far more fatalities every year than there actually are. (= Fleming did discover penicillin so there are fewer fatalities now.)