What would be the correct form of the verb in this sentence? They all have a noun clause a the subject followed by to be.
What you can do is playing with them.
What you can do is to play with them.
What you can do is play with them.
I always thought the first two are grammatical as it stands as a noun (object of the sentence), so it should be a gerund or infinitive. But recently I have encountered some instances of the third case. I also looked some similar structures up on corpus and found similar instances:
What you can do is take seeds from the crop that the seeds grew and then use them. (independent.co.uk)
What you can do is assess the risks and hazards and set up ways of reducing risk. (guardian.co.uk)
If the third one, using bare infinitive, is syntactically correct, how do you justify it? Is it considered an adjective of sorts here?