A subordinate clause is a sentence that can't stand on its own because it starts with a special type of word called a "complementizer," and therefore is part of another sentence. In this section, we will study the complementizers.
A Subordinate Conjunction is for the simplest of subordinate clauses, and you will use it the most. Use it for making one sentence the direct object of a verb. In English, this sort of sentence normally goes like this:
"I know that Jimmy fell into the well."
"That Jimmy fell into the well" begins with the complementizer "that," and therefore is a subordinate clause.
Be careful. In English, we often leave "that" out of the sentence, as seen in "I know Jimmy fell into the well." If you can insert a "that" and the meaning is unchanged, then you know that you have a subordinate clause on your hands.
The Sindarin subordinate conjunction is i. We don't know if you should mutate the word following it. Sometimes I do, sometimes I don't to distinguish it from the relative pronoun.
Iston i Amathon ananc i orch. - I know [that] Amathon slew the orc.
Eru amarthast i adanath firir. - Eru decreed that all humans die.
As you can see, in the past tense, a verb with this sort of subordinate clause is conjugated like an intransitive verb.