- Referencias (a las cláusulas):
•  Present simple.
•  Present continuous.
•  Past simple.
•  Future.

Ahí les va un par de ejemplos…

I /play/ tennis at this moment.
I am playing tennis at this moment.
x I am not playing tennis at the moment.
? Are you playing tennis at this moment?
She /eat / a hamburger now.
 She is eating a hamburger now.

x She is not eating a hamburger now.
? Is she eating a hamburger now?

He /phone/ his friends next week.
He is going to call his friends next week.
x He is not going to call his friends next week.
? Is he going to call his friends next week?
They /go/ to the cinema next Saturday.
They will go to the cinema next Saturday.
x Will they go to the cinema next Saturday?
? They won’t go to the cinema next Saturday.

The teacher /explain/ the lesson last class.
The teacher explained the lesson last class.
x The teacher didn’t explain the lesson last class.
? Did the teacher explain the lesson last class?
I /win/ a competition in 2009.
I won a competition in 2009.
x I didn’t win a competition in 2009.
? Did you win a competition in 2009?

She usually /phone/ her friends.
She usually phones her friends.
x She usually doesn’t phone her friends.
? Does she usually phone her friends?

They /surf/ the Internet every weekend.
They surf the Internet every weekend.
x They don’t surf the Internet every weekend.
? Do they surf the Internet every weekend?

• Pasado de:
Do / does  did.
Be (am – is – are)  was (I – he – she – it) / were (I – you – we – they).