Learn Dutch: Deze zin is moeilijk. - This sentence is difficult.

In Dutch, deze (this) is used for de-words, and zin is a de-word (de zin). If you were pointing out a het-word, like het woord (the word), you would use dit, as in: Dit woord is moeilijk.

Since zin is a de-word, why doesn't moeilijk get an extra -e at the end?

We only add an -e to the adjective when it comes directly before the noun (like de moeilijke zin meaning "the difficult sentence"). When the adjective comes after a verb like is, it keeps its basic form without the extra -e.

How is the -lijk part of moeilijk pronounced?

Even though it is spelled with an ij, the suffix -lijk is pronounced like "luk" (with a short, unstressed "uh" sound). So moeilijk sounds roughly like "MOO-y-luk".

Does the word zin always mean "sentence"?

Not always! While it means "sentence" here, zin is a very common word with other meanings. It can also mean "sense" (as in Dat heeft geen zin - That makes no sense) or "desire/mood" (as in Ik heb er zin in - I am looking forward to it).