Learn German: Ich wusste, dass dieses Programm Unsinn ist. - I knew that this program is nonsense.

Adding the prefix un- creates the direct opposite of a word. Here, der Sinn (sense or meaning) becomes der Unsinn (nonsense). When you add un- to a noun, the gender always stays the same, so it remains a masculine noun.

Since the first half says Ich wusste (I knew), why does the sentence end with ist instead of war?

In English, we usually shift the tense to match the past: "I knew that it was nonsense." In German, if the fact you are talking about is still true in the present (the program is still nonsense), you typically keep the second clause in the present tense (ist).

Why use wusste instead of kannte for "knew"?

German has two main verbs for "to know." You use wissen (past tense: wusste) when you know a fact or a piece of information—very often followed by a dass clause. You use kennen (past tense: kannte) when you are personally familiar with a person, place, or object.

Why does the verb ist go at the very end of the sentence?