"an argument is valid when if its premises are true, its conclusion must be true"
yet, "some valid arguments contain only false propositions"
how? i'm not fully understanding how it is the case that an argument can be valid even if it contains only false propositions when the definition of a valid argument states such.
me understand this concept(s).

"An argument is valid when if its premises are true, its conclusion MUST be true"...
is trying to define a true argument.

The conclusion of an argument could be false, but the argument remains true.
Here is an example.  We define the following propositions.
p: ABC is a right triangle
q: one of the angles A, B or C is a right angle.

Argument: p -> q
(interprets if p is true, then q MUST BE true)
p is the premise, q is the conclusion.
The argument says that if p is true, then q must be true.
=>
If triangle ABC is a right triangle, then one of the angles A, B or C MUST BE a right angle.  [ if premise is true, conclusion MUST BE true]
However, if ABC is NOT a right triangle, the conclusion may or may not be true, i.e. does not have to be true.   So even if the conclusion is not true (because the premise is not), the argument p->q remains valid.

Please think about it and post if further explanation is required.