At first glance, You always dress in a black suit looks pretty equivalent to You are always dressed in a black suit, with the apparent meaning being "You always wear a black suit". A number of people commented that they are the same. In fact, both constructions are in common usage. You might even hear cases of the constructions being used interchangeably. However, they are actually different, and when used properly, they are used differently, and with a different nuance in meaning.
The main source of confusion is in viewing the difference between the sentences as relating to different tenses of the verb "dress". In fact, there is discussion in the comments that "are dressed" is simply passive construction. I'll explain how and why the sentences are different, and how the usage is different when speakers are not being sloppy with the language. [1]
The gist of the difference stems from this: you dress... describes a habit, you are dressed... describes a present example or observation of the habit.
Tense
Lets focus first on tense and consider a simplified example:
- You were dressed in black explicitly describes something in the past.
- You are dressed in black explicitly describes something in the present.
- You will be dressed in black explicitly describes something in the future,
- You dress in black is simple present, which is not anchored to the present time. "The principal use of the simple present is to refer to an action or event that takes place habitually." (See Wikipedia discussion)
You always dress in a black suit
Simple present "dress" is the verb, and this describes your habit. It would be said by someone familiar enough with your routine to know that it's what you always do. It could refer to what you're currently wearing, but it could also be said in a context that doesn't involve that person actually seeing you, and it could refer to any time frame.
For example, you could be having a telephone conversation with the person discussing what you should wear at an upcoming event.
You: "What should I wear?"
Other person: "You always dress in a black suit."
That's a comment about your habit. The meaning would depend on the context of the discussion and the intonation of the words. It could be a suggestion to wear what you usually wear, or it could be a suggestion to wear something other than what you usually wear.
Notice that this construction has no inherent positive or negative connotation. In addition to the positive and negative examples above, it can be used with no value connotation at all, simply a statement of fact.
You are always dressed in a black suit
As similar as this sentence seems, it is actually a different beast. I'll break it down. Consider this sentence:
You are tall.
"Tall" is an adjective describing "you". "Are" is the verb. Now replace "tall":
You are [dressed in a black suit].
In this construction, "dressed" is not the past tense of the verb "dress", it is an adjective, part of an adjective phrase that describes "you". By coincidence the Macmillan Dictionary definition of "dressed" as an adjective uses this as an example: "She was dressed in a black suit." What are the odds of that coincidence?
So "dressed" does not contribute a tense, it is not a past participle verb, and it is not part of an "are dressed" passive construction. The only verb in the sentence is "are", and as in the discussion under Tense, it explicitly describes something in the present. The present is one example, or an observation of one example, of the habit.
"Always" indicates that it is something recurring and routine, as in "You always say that". "Always" in combination with "are" refers to a recurring pattern or series of then-present events. These are examples or observations of the habit.
This is the perspective of an observer. The observer sees a collection of then-present examples, which for the observer, is a pattern of observations. This construction is a generic reference to the observations of the habit rather than the habit, itself.
It would most likely be said by someone observing you and commenting on your present attire. It would typically mean:
Every time I see you, you are wearing a black suit.
It is a reference to their observations of you rather than to what you wear when they don't see you.
This construction is different from simple present in terms of the usual positive/negative connotations of its use. It is rarely used with a positive connotation. It can be used in a neutral way, but it tends to be used with a negative (or sarcastic) connotation.
Going back to the telephone discussion of attire for a future event, this construction could also be used there, but in a slightly different context. It would also reflect the perspective of the observer more than your habit. The two constructions:
You: "I think I'll wear a black suit."
Other person: "You always dress in a black suit."
This is a reference to your habit and a suggestion to change it for this event.
You: "I think I'll wear a black suit."
Other person: "You are always dressed in a black suit."
The perspective of this comment is that a black suit is what other people always see you in and a suggestion to let them see you in something different.
[1] Many thanks to Ben Kovitz for identifying the language principles underlying the "why".
Best Answer
If you are speaking literally, "in" usually carries the meaning of "surrounded by", or "completely enclosed":
and "on" implies "attached to" or "touching":
But it's a little bit trickier when you are speaking figuratively, using idiomatic expressions, or discussing abstract ideas where the physical concept of "completely surrounded" vs. "merely touching on the outside" doesn't really apply.
When it comes to set expressions like "on the bus", "in synch","on the team", or the cases you mention, "on the test" or "in this chapter", often these just have to be learned by rote.
Not to mention confusing pairs like:
and
Many of these are not subject to reason at all, just custom.
Sometimes it will even vary by region. Where I come from, people "stand in line", but my husband always refers to people "standing on line". He claims that's the way they say it in New York City.
Another example is the comment of Ronald Sole above, where he says, "In the test is more common than on the test." But in the variety of English spoken where I live, (California, USA), it is just the opposite. We would be much more likely to ask, "How did you do on the test?" than "in the test."