The word yIn presented alone and without context might be translated as the dictionary entry "live" or as a complete sentence "It lives." In this case, the presence of the period indicates that we are using it as a sentence and not a simple dictionary entry. Make sure to watch for those periods.
You would use the appropriate verbal prefix -- yIyIn! when speaking to one person, peyIn! when speaking to more than one person.
A handful of commands are taught in early lessons where the forms are simply to be memorised; then later on, there is a unit which explains imperatives (command forms) in general and practises them.
There is actually a difference in Klingon, but it's a difficult distinction to make. Klingon verbs seem to come in three varieties, but only the third one is distinguished in dictionaries.
The first category is transitive verbs. These can take an object, but can also be used without an object. These are the most common kind of verbs and include things like legh "see", yaj "understand", Sop "eat", etc.
The second category is intransitive verbs. These cannot take an object. We do not have a clear list of these and can only make supposition based on the English definition and/or on usage. When Dr. Okrand has been asked about transitive vs. intransitive he has responded with something like "Klingon doesn't have those concepts." None the less, there are words that appear to not be able to take an object, like qet "run", Qong "sleep", Hagh "laugh", etc.
The third category is qualitative or stative verbs. These are not really an action, but rather a state or quality that the subject could have or be. These cannot take an object and can be used like adjectives by placing them after the noun they are describing. These words can be identified in the dictionary because their definition begins with the word "be": val "be smart", Qup "be young", Quch "be happy", etc.
yIn is defined as "to live". We have some evidence that it can take objects if that object is something that can be lived (like "a life"). It definitely cannot be used as an adjective. In English there is little difference between the sentences, "it lives," and, "it is alive," but I am hesitant to call them the same, since "to be alive" sounds like a qualitative/stative verb and yIn is definitely not one of those.