When to use a gerund or an infinitive after "is"?

How does one know when to use a gerund or an infinitive? states a 90% rule, but I'm more interested in the remaining 10%.

This British Council page states

Sorry, there isn’t a rule. You have to learn which verbs go with which

pattern.

and then lists some verbs, after which the correct form applies.

But what about these two examples (if I got it right)?

My dream is to become a doctor.

My hobby is listening to music.

Why does one take a gerund, and the other an infinitive? What is the rule?

Answer

Unfortunately, the linked answer is very vague, and not correct. It does point out correctly that gerunds are more common as subjects than infinitives. But it certainly doesn't provide any rule that works.

The British Council is right.
It depends on the predicate in every case, and often both are OK.

The examples given are both correct, and illustrate a difference between infinitives and gerunds:

• My dream is to become a doctor.
  (to become a doctor describes a punctual result, with a changed situation as an outcome)


• My hobby is listening to music.
  (listening to music describes a generic durative activity, an experiential event)

Infinitives are more complex than gerunds, and they roughly point to different kinds of sentence:

As McCawley 1998 puts it (p.126)
Roughly speaking,
• that-complements correspond to propositions
• for-to complements [infinitives] correspond to situation types
• 's-ing complements [gerunds] correspond to events

This is the reason why infinitives are less common as subjects. Events are causes, and causes are old information; situations are normally results, and therefore new information. And the English sentence normally presents the old information first and the new information last. That's not a grammatical rule -- it's just the usual tendency.

And in this case, note how the sentences fare when reversed:

• ??To become a doctor is my dream.



• Listening to music is my hobby.

The predicate be X's dream here refers to X's personal desire for a particular situation, rather than an activity that X participates in. The opposite is true for the predicate be X's hobby, which specifies an activity, but no result. So the infinitive is uncomfortable as a subject in the first sentence -- it's result, not cause; but the gerund is fine.