activities online:
learnenglish.britishcouncil.org
ego4u.com

1. If the verb can be used with or without TO
(and with or without an object before the infinitive)
when we use it without TO,
we sound more informal.

Could you HELP me TO look for my car keys? I can't find them anywhere.
Could you HELP me look for my car keys? I can't find them anywhere.
Would you like to HELP TO cook dinner tonight? It's late and I'm feeling tired.
Would you like to HELP cook dinner tonight? It's late and I'm feeling tired.

There are one or two other structures where to-infinitive and the bare infinitive are both possible. Expressions with do or did, such as what I've done or all I did can follow either pattern.

I hate shopping so what I've done is (to) order a new computer over the Internet.
All I did was (to) suggest that she should lend him no more money. I didn't insist on it.

2. If two infinitive structures are connected
(by and, or or, except or but and than or as )
it is normal to omit to in the second clause.

I would like you to tidy the house and (to) wash the dishes before I get home.
Would you prefer to have a snack now or (to) wait until later before we eat?
I could find nothing to do this afternoon, except read my book.
My son does nothing but watch TV when he gets home from school.
It's quicker to bike to the station rather than take the car.
I have to fix breakfast for everybody as well as take the children to school before I can leave for work.

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source: bbc.co.uk

The Infinitive with TO

The Infinitive without TO

after auxiliaries/modals

can	He can run very fast.
could	As a boy he could run very fast.
may	I may fly to Africa this summer.
might	I might fly to Africa this summer.
must	I must go now.
mustn't	You mustn't smoke here.
needn't	You needn't go.
shall	We shall sing a song.
should	We should sing a song.
will	She will cook a meal for his birthday.
would	She would cook a meal for his birthday.

after to do

do	I don't know.

after the following expressions:

had better	You had better clean up your room.
would rather	Susan would rather study for her exam tomorrow.
would sooner	I would sooner read a book than watch this film.
why not	Why not ask your neighbour for help?
why should we	Why should we go by car?
why should we not	Why should we not go by car?

after verbs of perception + object (action has finished):

feel	She feels the rain fall on her face.
hear	I heard Peter sing a song.
notice	Mandy noticed the boy climb the tree.
see	They saw him climb up the roof.
watch	He watched the thieves steal a car.

after let + object:

let	Sandy let her child go out alone.
let	Mother let her daughter decide on her own.
let's	Let's go for a walk through the park.

after make + object:

make	She made Peggy and Samantha clean the room.

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source: englisch-hilfen.de

INFINITIVES: with or without TO