"The word 'cat' is a noun."
Translation:Η λέξη γάτα είναι ουσιαστικό.
When do you use the word λέξη, and when the word λόγος? The latter seems to be wrong here?
λέξη is a word as in a vocabulary item -- you would use it when you are saying things such as "this word has five letters and two syllables", "this word is masculine", "there are 1200 words in this essay" etc. You're not really concerned about the meaning of the words; you're treating them more or less as objects.
λόγος is more for words as meaning, and is most common in the plural -- τα λόγια του "his words", for example, to refer to a speech. You're not saying that his words had three syllables each (treating them as objects) but saying that his words were persuasive (treating them as meaningful content).