Question

1. I knew when he would come back.
(What is the part of speech of 'when' in the sentence? Is it an interrogative adverb or a relative pronoun?)

2. I knew the time when he would come back.

(In this sentence 'when' is a relative pronoun, right?)

3. I know ___? + When will he come back?

--> I know when he will come back.

(If the two expressions are combined and make the sentence, 'when' can be regarded as an interrogative adverb. Right?)

4. I know the time when he will come.
(In this sentence, 'when' will be omitted. Then 'when' can be regarded as a relative adverb, right? as in the sentence" I know when he will come.)

Answer 1

1. The word "when" is a subordinating conjunction. It introduces the subordinate clause ("when he would come back"). [The main clause is "I knew."]
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

2. Yes. It introduces the clause "when he would come back" which is serving as an adjective -- the whole clause modifies the word "time."
http://owl.english.purdue.edu/owl/resource/645/01/

3. No -- see #1 above.

4. See #2 above. It's the same construction even though the words are slightly different.