"I believe you."
Translation:Je vous crois.
"Tu" is the subject of a sentence: "Tu portes un manteau". The verb is happening to the subject. "Te" is an indirect object, in this case the verb happens to someone/something else besides the subject. Ex: "Je parle à toi" (I talk to you). In French the indirect object "à toi" gets shortened to "te". See: http://french.about.com/od/grammar/a/indirectobjects.htm.
There is no "je tu crois". When you want to say "I believe you", "you" is the object and you must use an object pronoun. It may seem odd because in English the subject pronoun and the object pronoun are both "you", but in French the (informal) object pronoun is "te" and the subject pronoun is "tu". The link shown by Katie_B. explains it.
No, you can't because "croire" is directly transitive (direct object), so the form "te" has to be used, and placed in front of the verb.
"toi" is the stressed pronoun of "tu", used mainly as a multiple subject, after a number of prepositions (de, par, pour, sans, avec, etc) or as an apposition:
toi et moi allons à l'école
je vais à l'école avec toi
toi, tu es un bon étudiant
I know the difference between direct object, indirect object and stressed pronouns; thankfully, so far, I'm doing fine with that.
but I remember hearing in a song, "crois moi", so I started to wonder.
- is that an imperative form of speech?
if so, can we still say?
crois-toi/vous don't make sense to me, even if they were valid
With the imperative voice, this is the pattern:
crois-moi / le / la / nous / les (fem & masc).
The reason why it is not crois-me is phonetic, you would not make a final "m" sound audible enough (my guess).
For the other persons, the direct object form agrees with the construction of verb "croire".
(tu/vous don't make sense, right)
truth be told, you just lost me
I remember reading on about.com once that me & te change sometimes to moi & toi, no reason was mentioned though; so I guess it might be as you said, just to make the flow of the speech easier.
though, I don't know why we should use direct object pronouns rather than stressed pronouns, can you please explain?
What I said is that in the imperative, only "moi" has its stressed form. All others are in the direct object form.
Stressed pronouns have very specific uses, as I said before.
To use them as regular direct object pronouns is not correct.
tu me crois - crois-moi !
tu la crois, tu le crois - crois-la !, crois-le !