Imperator non laudat Gallios.

(the verb has to be singular, and the gauls have to be in the accusative case)

2. The Gauls do not praise the laws of the romans.

Gallii non laudant leges Romanorum.

(The plural is Gallii with two Is rather than one. The laws have to be plural and accusative case, while “of the Romans” means it has to be plural and genitive case.)

3. Caesar is the leader of men.

Caesar dux hominum (est).

(You don’t have to do anything about the word dux, just leave it in its original nominative case form. You can think of it as an equation: Caesar = leader, both are nominatives, so no changes. and the is/est is optional, Romans weren’t very keen on using forms of “be” unless they wanted to stress a certain point.)

4. The leaders of the Romans are in Gaul. And so the kings of the Gaul are servants of the Romans.

Duces Romanorum in Gallia sunt. Et tam reges Galliorum servi Romanorum sunt.

(If you put dux into the nominative plural form, you get duces. “Of the Romans” means it’s genitive case again, thus Romanorum. Forms of “be” are usually put at the end of a Latin sentence. You forgot to translate “so”, which is “tam”, then you have to put rex into the nominative plural form to get reges, just as slaves becomes servi. And the other two forms are both genitive plural, so they have to be Galliorum and Romanorum respectively.)

5. The general sees the dangers of war.

Imperator videt periculos belli.

(It has to be periculos because it’s the accusative case, and belli because it is genitive.)

6. Marcus does not praise the glory of war

Marcus non laudat gloriam belli.

(once again, it’s the accusative case (whom/what does he not praise?), so gloria becomes gloriam. And again, you’ve got war in the singular genitive form, and that’s belli.)