Cleft Sentences with "what"
1)Resorts shouldn't take too many people.
(should/impose/restrictions)

Correct answer: What they should do is impose restrictions.
(Cleft sentence without "to")

2)I'm sorry but the plane is going to be delayed.
(can/offer/meal vouchers)

Correct answer: What we can do is to offer you meal vouchers.

Another example with "to" in a cleft sentence:
What I'd really like is to lie by the pool.

My question:
I couldn't found a rule when I have to use " to + verb" in a cleft sentence. However cleft sentences are mostly indicated without a "to", like in the first example (Nr. 1)

Can anyone answer my question? Thank you.

Question

Cleft Sentences with "what"
1)Resorts shouldn't take too many people.
(should/impose/restrictions)

Correct answer: What they should do is impose restrictions.
(Cleft sentence without "to")

2)I'm sorry but the plane is going to be delayed.
(can/offer/meal vouchers)

Correct answer: What we can do is to offer you meal vouchers.

Another example with "to" in a cleft sentence:
What I'd really like is to lie by the pool.

My question:
I couldn't found a rule when I have to use " to + verb" in a cleft sentence. However cleft sentences are mostly indicated without a "to", like in the first example (Nr. 1)

Can anyone answer my question? Thank you.

Answer

First, I think the second sentence is wrong. It should be, "What we can do is offer you meal vouchers."

Whether to use "to + verb" or not depends on whether a verb or noun needs to come after "is."

If a verb [i.e. action] needs to come after "is," don't use "to."
EXAMPLE: What they should do is _________ [they need to do an action/verb]
                   What they should do is impose restrictions. [impose = verb]

If a noun [i.e. thing] needs to come after "is," use "to + verb."
EXAMPLE: What I'd really like is _________ [noun]
                   What I'd really like is a book. [book = noun]
                   What I'd really like is a nap. [nap = noun]
                   What I'd really like is to lie by the pool. [to lie = noun]

REASON: "to + verb" is called an "infinitive." Infinitives are verbs that act like nouns.