"That is a heavy knife."
Translation:taj 'ugh 'oH.
Isn't taj supposed to be tajvetlh, since we are talking about That knife?
Well, the English sentence doesn't say "that knife", but your question exactly addresses the purpose of this sentence, so let me explain.
In English the word "that" can be used as a determiner or as a pronoun. As a determiner, it can be attached to a noun as in "that knife". Klingon has a similar grammatical structures using the suffix -vetlh.
As a pronoun, "that" can appear alone as a complete noun standing in for another noun, as in "that is". Klingon has no direct equivalent for this. People will sometimes use creations like Dochvam ("that thing") or just pick an existing pronoun (like 'oH) or even turn the sentence around so they can use -vetlh on the noun ('ugh tajvetlh).
Note that this English sentence doesn't say "That knife is heavy" ('ugh tajvetlh), but rather "That is a heavy knife." In this case the "that" is acting as the noun, not attaching to a noun. The purpose of this exercise is to get you to recognize this and begin to develope strategies to deal with that.
By the way 'ugh tajvetlh is accepted as a translation for this sentence.