Main clauses are grammatically correct full sentences that can stand alone.
Subordinate clauses, on the other hand, can't stand alone. They are missing something that is required in a full sentence.
Because German uses cases, word order is more flexible than in other languages. There are some rules you need to follow, though.
In German, the verb always has to go in a specific position.
In main clauses, the conjugated verb goes in position 2.
Exceptions are yes/no-questions and the imperative: the conjugated verb goes in position 1.
What about when there's more than one verb? Look at the lesson on sentence brackets.
You can put almost anything in position 1 in German:
* the subject is marked in green
In German the subject should be as close to the beginning of the sentence as possible. However, if something else is more important and should be emphasized, that should go in position 1.
Unlike in English, the subject does NOT have to come before the verb.
A: „Ich habe mein Handy verloren.“
(The subject is the most important thing)
A: „Wo ist mein Handy?“
(The question word is the most important thing)
B: „Auf dem Bett liegt es doch!“
(The answer to the question is the most important thing)
Main clauses are full sentences that can exist on their own.
The conjugated verb goes in position 2 in most sentences. The exceptions are yes/no-questions and commands (then it goes in position 1).
The subject goes towards the front of the sentence, but doesn't have to be in position 1 or even in front of the verb.
In my lesson on sentence brackets I explain where you should put verbs when there's more than one in the sentence.
How to form a grammatically correct question.
In addition to main clauses, there are also subordinate clauses.