Do I need to put a comma?

Question

Do I need to put a comma in this phrase "Исходя из этого можно понять(,) насколько сложно..."?

Answer 1

Actually, two commas must be put in this sentence:

Исходя из этого, можно понять, насколько сложно... .

The first comma is because phrases introduced by the preposition исходя из are separated by commas if they are not a part of the predicate.

The second comma is because clauses introduced by насколько are separated by commas in Russian.

Answer 2

Yes, you have to put a comma in this phrase because the clause, introduced by "насколько" needs a comma, being a separate close with a subject and a predicate.