title_body: What is the difference between tense and aspect? Many conlangs contain tense, some contain aspect as well or instead, and much of the recommended reading for conlangers assumes that you know what tense and aspect are and what the difference between them is. What actually is the difference, and how would a word or morpheme be classified as a marker of one or the other?
upvoted_answer: When discussing tense, aspect, and mood, it's important to distinguish a given language's grammatical markers from the abstract concepts being described. Thus, linguists use the words temporal reference and aspectual reference to describe the abstract ideas being described, while the words tense and aspect are reserved for when such reference is marked grammatically. 
  
  The simplified explanation for tense and aspect usually given is that tense describes when an event occurs and aspect describes the internal temporal structure of an event. However, this doesn't give a ton of easy answers to "is this grammatical form marking tense, aspect, or both?" Formal semanticists actually have a relatively simple framework for how temporal reference and aspectual reference are defined, the basic concepts of which can be helpful when designing your conlang's own tense/aspect system.
  
  The Neo-Reichenbachian Model
  
  Imagine a timeline:
  
  Earlier                                  Later
  &lt;--------------------------------------------&gt;
  
  
  Now, imagine there are two spans on that timeline (the fact that they're spans is important -- nothing is truly instantaneous). The time when you make an utterance is called the utterance time (or UT), and the time that you're talking about is called the topic time (or TT). 
  
  Temporal reference describes the relationship between the TT and UT. Past temporal reference is when the TT precedes the UT; future temporal reference when the TT follows the UT; and present temporal reference when they overlap. 
  
  Past:
      &lt;--------------------------------------------&gt;
                  |--TT--|          |--UT--|
  
  Future:
      &lt;--------------------------------------------&gt;
            |--UT--|                  |--TT--|
  
  Present:
                              |--UT--|
      &lt;--------------------------------------------&gt;
                       |-------TT-------|
  
  
  This seems pretty intuitive -- if the section of the timeline you're talking about precedes the time you make the utterance, you'll use the past tense; if it follows it, you'll use the future; and if they overlap, you'll use the present. Simple, right?
  
  But what of aspect? Well, aspectual reference isn't as transparent at that. To describe aspectual reference, we'll need to pick out a third span that timeline -- the time when the event being described actually happened. We'll call this the eventuality time (or ET).
  
  "But wait!" you might object. "Isn't that the same as the topic time? If I'm describing something that happened, surely the time I was talking about is the same as the time that it happened." However, there is a difference, and it's exactly this difference that gives rise to aspectual reference. Consider the English past perfect in a sentence like the following: 
  
  
    Yesterday he had gone to the store.
  
  
  The English past tense includes past temporal reference, and we've established that this means that the topic time is before the utterance time -- something which is explicitly confirmed by the temporal adverb "yesterday" giving us a more specific topic time. But when does the him-going-to-the-store happen? Probably not yesterday, right? Well, this is because perfect† aspectual reference is when the eventuality time precedes the topic time -- aspectual reference describes the relationship between the eventuality time and the topic time. It for this reason that the past perfect is used by English speakers for the past-in-past.
  
  Past Perfect:
      &lt;--------------------------------------------&gt;
            |--ET--|    |--TT--|     |--UT--|
  
  
  But you can also have the present perfect, right? Well, let's combine our descriptions of the present tense and the perfect aspect and see what we come up with.
  
  Present Perfect:
                              |--UT--|
      &lt;--------------------------------------------&gt;
          |--ET--|     |-------TT-------|
  
  
  Here, the topic time and utterance time overlap, which is why you can't say something like "Yesterday he has gone to the store." However, the eventuality time precedes the topic time, so you're describing an event that happened in the past in relation to the present. And indeed, this is how we see the present perfect used in English.
  
  This is far from the most interesting thing aspectual reference can do, however. Let's discuss the popular aspectual references, perfective and imperfect. For those who don't know, perfective and perfect are two different things. Perfective is often described as simply viewing an event as a point on a timeline, and imperfect as viewing it as a span. How do we define this as a relationship between eventuality time and topic time?
  
  Well, a simple way people do it (ignoring a few other related thorny semantic problems that few here will care about) is by defining the perfective as when the eventuality time is contained within the topic time and the imperfect as when the topic time is contained within the eventuality time.
  
  Past Perfective:
                 |--ET--|
      &lt;--------------------------------------------&gt;
          |-------TT-------|        |--UT--|     
  
  Past Imperfect:
                 |--TT--|
      &lt;--------------------------------------------&gt;
          |-------ET-------|        |--UT--|     
  
  
  This results, at least formally, in the same sort of perspective shift we try to intuitively describe as changing the event from a "point" to a "span".
  
  Note that tense and aspect vary between languages a lot, and even related languages can't be counted upon to have the same semantics here. After all, the French passé composé, while very similar in form to the English present perfect, does not share the same semantics (it's more of a perfective). Additionally, while a given language's system of tense and aspect will often combine a tense and an aspect in a certain inflection (such as the perfective past tenses in many European languages), this doesn't mean the two are wed cross-linguistically.
  
  In fact, I hope this answer has given some conlangers the tools to try and invent their own tenses, aspects, or combinations of the two that they haven't themselves encountered cross-linguistically. Be creative!
  
  In short:
  
  
  Temporal reference is the relationship between the utterance time (when you say something) and the topic time (the time relevant to the discussion at hand).
  Aspectual reference is the relationship between the topic time (see above) and the eventuality time (the time when the event being described actually occured).
  Tense is the grammaticalization of temporal reference.
  Aspect is the grammaticalization of aspectual reference.
  Things called "tenses" in natural languages (as well as in conlangs, to be honest) are often conflations of tense and aspect (and sometimes even mood, but we're not here to talk about that).
  
  
  
  
  †When I refer to perfect aspectual reference, I'm actually more specifically describing what is known in semantics as the existential perfect. Perfect forms in English and many other languages include other types of perfect aspectual reference (universal, resultive, etc.) that differ subtly in their semantics, and other aspectual and temporal baggage as well; this is, however, probably not particularly interesting or relevant to most people here at this moment.