What is the function of the noun clause in this sentence? Lam knew that she could control her breath.

The function of the noun clause is to describe the most important part of the sentence.

Explanation:

That's why the breath in this sentence is the noun, that show us what is the activity that the verb do, and what Lam can do.

So the noun are necessaries in all the sentence to explain what are the things that we do, with actions or verbs, and what we describe with adjetives.