“Was” or “were” in sentence where no noun subject exists

I can't figure out if the below sentence should use “was” or “were”, since I'm not sure what the subject of the sentence is. Is the sentence incomplete? How do I handle this kind of sentence, where no nouns or pronouns are present?

Knowing how to jump and being able to run was / were crucial during my high jump career.

Answer

1. Knowing how to jump and being able to run was / were crucial during my high jump career.

As to the subject of the OP's example sentence:

• The subject is the expression: [Knowing how to jump] and [being able to run].


• That subject is in the form of an 'and'-coordination.


• The coordinates are two '-ing' clauses (which are non-finite clauses).

If a subject is in the form of an 'and'-coordination of clauses, then the default for subject-verb agreement, w.r.t. number and person, is singular and third person; and that is what is often used. In the case of the OP's example, that would mean the use of the singular verb "was":

2. Knowing how to jump and being able to run was crucial during my high jump career.

Though, it is possible for the writer to use the plural verb "were" if that would be more appropriate due to what is in the rest of the sentence or due to the writer's intent for that sentence.

Here's a related excerpt from the 2002 CGEL. On page 508:

And-coordinations of clauses

Subjects with the form of an and-coordination of clauses generally take singular verbs:

[29]

• 
  

  i. [That the form was submitted on the very last day and that the project had not been properly costed] suggests that the application was prepared in a rush.

  
  


• 
  

  ii. [How the dog escaped and where it went] remains a mystery.

  
  

It is nevertheless possible to have a plural verb when the predicate treats the coordinates as expressing separate facts, questions, or the like:

[30]

• 
  

  i. [That the form was submitted on the very last day and that the project had not been properly costed] are two very strong indications that the application was prepared in a rush.

  
  


• 
  

  ii. [How the dog escaped and where it went] are questions we may never be able to answer.

  
  

Notice how the two examples in [30] have predicative complements that are noun phrases that are plural in number ("two very strong indications . . .", "questions . . .").

Also, notice that the CGEL examples in [29-30] use subordinate content clauses (which are finite clauses) for its examples, while the OP's example is using two non-finite clauses which just happen to not have subjects in them. (For a version that is similar to the OP's example but instead uses '-ing' clauses with subjects could be something like: "My knowing how to jump and my being able to run was crucial during my high jump career.")

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(Aside: In general, the default subject-verb agreement, w.r.t. number and person, for a clause can be considered to be singular and third person. Usually there has to be something explicit in the sentence for that subject-verb agreement to be otherwise.)

Note that the 2002 CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language.