When comparing English verb forms, the grammatical complexity of each form must be taken into account. Even where a clear semantic distinction can be made between two forms, such as the "simple present" and the "present progressive", the grammatically simpler of the two tends to be used (regardless of its semantics) in the absence of such a distinction. This is most clearly exemplified in the class of verbs known as "statives".
Statives can be considered verbs with no inherent punctuality. They describe states or properties rather than complete actions.
1) a. She has an apple. b. You're crazy. c. I like this town. d. They want to leave.
The vast majority of English verbs do not fall into this class and might instead be called "actives". These verbs denote punctual, complete events and therefore take on a habitual or predictive meaning in the simple present tense. If an event is viewed as a complete, unbroken unit, it can't really be occurring right at this moment, but it might occur repeatedly over a period of time that includes the present moment, or it might occur in the future.
2) a. I work at home. b. He drinks whiskey. c. They leave in the morning. d. We take the bus.
Notice the ambiguity in (2c). Actually, each of the above examples might be interpreted as either habitual or future events, depending on their context. Any active verb such as these can be used to express a stative meaning if the present progressive is used.
3) a. I'm working at home. b. He's drinking whiskey. c. They're leaving in the morning. d. We're taking the bus.
Ambiguity between present and future still exists in these examples, but with a subtle shift in meaning. It's easy enough to see this in the present interpretation; we're simply talking about "right now" as opposed to "generally". If these sentences refer to the future, the progressive form seems a bit more deliberate. The examples in (2) sound like the subject has little or no control over the event; it just happens that way. In (3), the subject has made a conscious decision.
The differences between the forms in (2) and (3), in both their present and their future interpretations, can be summed up as an external vs. an internal view of the events. Those in (2) are viewed from the outside as dots on a timeline, but in (3) we go inside the dot and view it from within. From this perspective, the events in (3) are always incomplete. What we're really talking about is more like a state than an event.
This "external vs. internal" analysis is supported by the fact that the simple present can be used in narratives without its usual non-present-moment interpretation. If you say, "A man sits down and orders a cup of coffee. He reads the paper as he waits..." each of these events seem to happen at the present moment, but the entire situation is viewed from the outside, hence the simple present.
But what are we to make of the following examples? If verbs that are already stative in the simple present can also occur in the present progressive, then what does this mean?
4) a. She's having an apple. b. You're being crazy. c. I'm liking this town. d. They're wanting to leave.
Actually, many stative verbs can occur in the progressive aspect, but their meaning changes, sometimes dramatically. In (1a), someone is in possession of an apple, but in (4a), she's eating it! These are very different ideas. As soon as a stative verb is put in the progressive, it seems to be converted into an active verb. The meaning of this new homophonous verb is typically more deliberate or incipient than its stative counterpart.
In addition to the sort of contrasting examples I've provided here, I think the difference between the simple present and the present progressive could be illustrated through exercises involving timelines. To chart a given event, a circle or ellipse would be drawn onto a blank timeline so that it covers the entire duration of the event. (In the case of a habitual or repeated action, there would be more than one circle on the timeline.) If we're viewing this event from the outside as a complete unit with both the beginning and the end in sight, then the circle should be filled in. Otherwise it should be left empty to symbolize our viewing the action from the inside.
Drawing these timelines might be a bit tricky for students, so more focus should be put on matching example sentences to their correct representations. The more context provided for these example sentences, the more obvious the right timeline choice should become. For active verbs, timeline circles associated with the simple present should always be filled in, and those associated with the present progressive should always be left empty. For true statives (and not their homophonous active counterparts), only empty circles should be used.
This timeline approach could also be used with another verb form whose interpretation and use might prove difficult to learners: the "present perfect". Consider the following examples.
5) a. I've worked at home. b. He's drunk whiskey. c. They've left in the morning. d. We've taken the bus.
The most typical usage of the present perfect in American English refers to an experience at some unspecified (and presumably unimportant) point in the past and often involves the adverb "before". I find the way this particular notion is expressed in Japanese to be quite elegant and enlightening.
6) Anata-wa ika-o tabeta-koto-ga aru? You-TOPIC squid-OBJECT ate-event-SUBJECT exists? Does the experience of having eaten squid exist for you? Do you have the experience of having eaten squid? Have you ever eaten squid?
I used so many intermediate translations here to illustrate how clearly and literally the concept is expressed in Japanese. (The word "koto" is really vague, almost like "thing" but only for referring to events, experiences, deeds, and that sort of thing.) If you have eaten squid, you literally "have" that experience. It's no accident that we use the word "have" in our expression of the same concept. This could be a useful memory hook when introducing the present perfect for the first time.
As we talk about having a given experience, we're still in the present tense because this past event is only a memory used to explain or comment on our present situation. This is true even for less common uses of the present perfect such as this classic:
7) I've fallen and I can't get up!
Just as the simple present can step into territory normally reserved for the present progressive, the simple past can step in here. In fact, (7) sounds a little stilted for modern American English. A more natural version of (7) would be:
8) I fell down and I can't get up!
Of course, as you introduce more and more verb forms to students of English, things will just get more and more complicated. For example, combining the progressive with the perfect, you get the following minimal set.
9) a. I work out. b. I'm working out. c. I've worked out. d. I've been working out.
If timelines are used to represent the events expressed in (9a-c), then the proper interpretation of (9d) should be fairly easy to predict. (9a) would involve filled circles scattered evenly across the timeline (beginning in the past and going into the future). (9b) would be an empty circle around the timeline's center marker. (9c) is unclear, but it could involve a filled circle or two in the past (and none in the future). (9d) should have several empty circles drawn right up to the present (but not past it).
In my experience, I've found this sort of explanation through charting events onto timelines to be effective and memorable. If the students don't share an L1 (or if the instructor doesn't speak this language), then a graphical explanation of this nature is pretty much the only option.