How to indentify "subject of verb" in the noun clause

The noun clause can be the subject of a verb in a complex sentence.

For example,

When she shall return is uncertain.
That it would rain seemed likely.
 
To identify the noun clause as the subject, ask a question with 'What'.

What is uncertain?
When she shall return. (Noun clause as the subject of the verb)

What seemed likely?
That it would rain. (Noun clause as the subject of the verb)