Expression “一...就...” - what if there is a sub-sentence? Example:   我回家以后就预习功课。
回答： 我一回家就预习功课。

This structure is quite easy. However, how would we apply the same “一...就...” structure if there is an extra 'sub-sentence' in the example (i.e. 这一课很容易).
What would be a grammatically correct answer then, to make it into a smooth whole?

Example: 这一课很容易，我看了一会儿就懂了。
回答： ...❓

Would 我一看这很容易的课就懂了。 be correct?
Would 我一看了这很容易的课就懂了。 be more correct?
Would 我一看这很容易的课就懂了。 be correct?

The sentence is grammatical, However, 这课我一看就懂 already indicated you think it is easy, adding &quot;很容易的&quot; make the sentence unnecessarily wordy.
You can add relative clauses in any part of the sentence as long as it makes sense:
这油画我一看就知是假的
这(标价三千万的)油画，我一看就知是假的
这油画(用的是二十世纪才有的油彩)，一看就知是假的
这油画，(身为画家的)我一看就知是假的 -