В пособии рассматриваются основные моменты английской грамматики: части речи имя существительное, имя прилагательное и наречие; видо-временные формы глагола; модальные глаголы; неличные формы глагола

certain, sure
certain/sure + infinitive means that the subject will definitely do it:
He is certain/sure to win the race.
(=the other sportsmen are far worse)
certain/sure + gerund means that the person feels certain or sure he will
do it, but he could be wrong:
Starting the experiment he was certain/sure of finding
the solution, but then he began to lose confidence.
In the first case we talk about what will happen, in the second we describe
a person’s state of mind.

interested
interested in + -ing refers to what will (may) happen. It means to be interested by the idea of doing something:
I’m interested in buying a house in the village.
(=I’m thinking of doing it, I’d like to do it)
interested + inf. refers to what has happened. The structure is usual with hear/see/know/read/learn, etc.
I was interested to know that your brother got the scholarship.

sorry
sorry + infinitive is used when we apologize for something that we are doing or about to do.
Sorry to interrupt you. I need your advice.
sorry + infinitive is also used to express sympathy or regret:
I’m sorry to hear that you’ve been ill.
sorry for + -ing is used when we apologize for something that we have done. In this case we can also use a perfect infinitive or a that-clause:
I’m sorry for ringing you up so late yesterday.
I’m sorry to have rung you up so late yesterday.
I’m sorry that I rang you up so late yesterday.

used
used + infinitive exists only in he past. It means that the subject did something regularly in the past but no longer does it:
They used to spend much time together but now they do not meet
very often.
be used to + -ing means that something isn’t strange or new for the speaker:
After living in the country for many years he is used to walking
long distances.