CONDITIONALS:

ZERO CONDITIONAL: Real situation.

If clause	Main clause
If + Present tense	Present tense
If you freeze water	it turns into ice.

If I work too much, I get tired. 
Si trabajo demasiado, me canso.

FIRST CONDITIONAL: True or possible situation.

If clause	Main clause
If + Present tense	will / can / may / must + verb
If it rains today,	I'll stay at home

If it rains today, I'll stay at home.

Si llueve hoy, me quedaré en casa.

SECOND CONDITIONAL: Hypothetical situation.

If clause	Main clause
If + Past Simple	would / could / might + verb
If I won the lottery,	I would travel around the world.

If I won the lottery, I would travel around the world.
Si ganara la lotería, viajaría por todo el mundo.

THIRD CONDITIONAL: Hypothetical situation last

If clause	Main clause
If + Past Perfect tense	would/could/might + have + past participle
If I had won the lottery,	I would have traveled around the world.

If I had won the lottery, I would have traveled around the world.
Si yo hubiera ganado la lotería, habría viajado por todo el mundo.

They write letters

*Letters are written by them

They wrote letters

*Letters were written by them

They are writing letters

*Letters were being written by them

She writes letter

She will write letter

*Letter will be written by her

She is going to write letter

*Letter is going to be written by her

She have written letter

*Letter have been written by her

They should read English books carefully at home every month
*When should they read English books carefully at home?
Simple Past:
They read English books carefully at home last week
*How did they read English books at home last week
Future with going to:
They are going to read English books carefullt at home tomorrow
*Where are they going to read English books carefully tomorrow?
Present Tense:
They read English books carefully at home today
*What do they read carefully at home today?
*Which books do they read?
Present Perfect:
They have read Ennglish books carefully at home since April
*How long have they read English books carefully home?