1

Here are several examples:

  1. Luis lost his job and was short of money, so what he did was (to) sell his flat and move in with his brother.
  2. The reason hundreds of students marched through the city centre was to protest against the new laws.
  3. What I want to do is (-) persuade them to come with us.
  4. What Jane did was (-) invest well.
  5. Our original motivation for developing TF-Replicator was to provide a simple API for DeepMind researchers to use TPUs.

I have difficulties in understanding when to between to be and the following verb:

  • is compulsory & required, as (I suppose) in the 2nd and 5th sentences above
  • is optional, as (I suppose) in the 1st sentence
  • is forbidden.
1

First Explanation

Here is the main source of the explanation below of when to use to, when not to use to.

Must use to

By default we must use to between any two verbs:

  • I want to sleep
  • He want me to buy
  • I was about to give up

Optional

After some verbs like help:

  • Could you help me to look for my car keys?
  • Could you help me look for my car keys?

In pseudoclefting in phrases like:

  • 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.

Must not use to

After modal verbs, including need.

After some causative verbs (to be precise, all of them, except get: My friends get me to take the test.):

  • She lets me borrow her book.
  • I will have someone cut my hair.
  • My English teacher has me do the homework in a week.
  • My mother made me do my homework., but not in passive(!):
    • I was made to cry a lot by the film.
    • A professional player would be made to shudder by these odds.

After the object after certain verbs, such as hear, see there is no to:

  • I saw him pour the medicine down the loo and I heard him laugh to himself.

After verbal idioms would rather and had better there is no to:

  • I'd rather swim in the pool than go down to the beach.
  • You'd better see what he wants.

Second Explanation

According to this tool, was invest is a type of VP (Verb Phrase):

             S
     NP                   VP
     NP                V    VP
NP        S           was   V
what     S/NP               V
       NP  VP/NP            V
       NP  V/NP           invest.
       he  did

Indeed, here is a relevant excerpt from Wiki article on VP and Pseudoclefting:

Dependency grammars point to the results of many standard constituency tests to back up their stance. For instance, topicalization, pseudoclefting, and answer ellipsis suggest that non-finite VP does, but finite VP does not, exist as a constituent:

...

These data must be compared to the results for non-finite VP:

  • ...and finished the work, John (certainly) has. – Topicalization
  • What John has done is finished the work. – Pseudoclefting
  • What has John done? – Finished the work. – Answer ellipsis

PS

There is an unanswered question, though - is there any difference in style (more formal / informal, for example) between using to and not using to in pseudoclefting.

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Use "to" with gerunds.

You use "to" when you are using a verb as a gerund, which is when a verb functions as a noun or as part of a phrase that acts as a noun. For example:

Luis lost his job and was short of money. To sell his flat and move in with his brother became his only options.

You can also make gerunds without using "to."

Not using "to" as part of a gerund is a stylistic choice. This version of the sentence features two gerunds ("selling" and "moving") that do not use "to."

Luis lost his job and was short of money. Selling his flat and moving in with his brother became his only options.

Some infinitives can be bare infinitives

Certain verbs will never use "to" in with the infinitive: *will, would, shall, should, may, might, can, could and must. This also applies to had better, would rather, sooner than, and rather than.

A number of other verbs may be bare infinitives in certain uses. This seems usually is done when certain verb relates to the action of the object of the sentence (not the subject) but not all verbs follow this. Examples of such verbs include (but are not limited to): bid, let, make, see, hear, need, dare, sit, cry, go.

As an example of an infinitive using "to" and not using it:

She made him cry. (Object is doing the crying and "to" is not used)

She started to cry. (Subject is doing the crying and "to" is used)

  • I perfectly understand what those sentences mean, I also perfectly understand your 2nd and 3rd examples. What I do not fully understand is when in general I must use "to" between two verbs, when I must not and when it is up to me. – DimanNe Apr 23 at 21:02
  • @Diman I've revised my answer. I hope you find it a little more useful now :) – rpeinhardt Apr 23 at 22:29
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This is the to-infinitive form of the verb (or verb phrase)

In English, many verbs require another word to indicate the tense of the verb. The "to" in this case indicates the infinitive of the verb phrases in question. This has many uses in English.

See Wiki Entry

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