I find that a very common confusion when first learning any language is the confusion of nouns and verbs. The best thing to do is to make sure the student has a firm understanding of parts of speech in English. This way, when the confusion arises you can ask the student what function the word serves in the sentence. Also, many times students try to apply verb rules to nouns and visa versa. When first learning a language (especially Latin), it is important to focus on each word in the sentence and understand its function and why it looks the way it does (ie. endings for verbs, nouns, adjectives). These are easy to correct when caught early, but once it's an engrained habit, it is much more difficult to address.