"This issue is very serious."
Translation:Bu sorun çok ciddi.
This is because, in this sentence, "this issue" (bu sorun) is the subject. Basically, the accusative is used to indicate a definite direct object, whereas the nominative is used for the subject. In other words, the accusative is used when you are specifying something that is being acted upon.
So, for example:
First, we'll let the bread be the subject:
"Bread is coming."/"The bread is coming."/"This bread is coming."
In all three cases, bread is nominative, since it is the subject which is performing the action. So,
"Ekmek geliyor."/"Ekmek geliyor."/"Bu ekmek geliyor."
(You'll notice, "bread" and "the bread" are the same in Turkish, as there's no immediate difference between an indefinite and definite subject in Turkish.)
Next, we'll use bread as the object:
"This dog is eating bread."/"This dog is eating the bread."/"This dog is eating this bread."
"This dog" is the subject; it is the noun which is performing the action described by the verb ("eats").
Now, bread is the direct object in all three cases, but there are slight differences:
In the first example, "bread" is indefinite. It's not specifying which bread, it's just some bread. So,
"Bu köpek ekmek yiyor.";
"ekmek" is not in the accusative, because although it's being acted upon (by this dog), it's not specific.
In the second example, "bread" is definite. It's specifying, "you know, the bread. The bread that we just brought home. The bread. You know which bread." So,
"Bu köpek ekmeği yiyor.";
"ekmeği" is in the accusative, because it's a specific thing being acted upon.
In the third example, "bread" is still definite. It's specifying, "this bread, the stuff I'm pointing at right now. This, over here, this bread." So,
"Bu köpek bu ekmeği yiyor.";
"[bu] ekmeği" is again in the accusative, because it's a specific thing being acted upon.
So, as you can see, while "[bu] sorun" is specific in this sentence ("this issue—this one, not that one nor that other one"), it is not an object, and therefore cannot be accusative.