Prolegomena

Let's face it: deponent verbs are peculiar. There are nothing like them in English. However, oddness aside, there's nothing difficult about them. Just bear in mind their distinctive feature:

Deponent verbs are passive in form but active in meaning. (By "form" we mean that the endings are all passive.)

In other words,

Deponent verbs look passive but are translated as though active.

The good news is that deponent verbs are identical in every respect to the passive forms of regular verbs. So, if you can recognize a passive form in a regular verb, you can recognize any deponent verb. (Of course, you may feel a little hazy about identifying any kind of passive verb-form...but that's another issue!)

It all boils down to knowing which verbs are deponent, and which are regular. That distinction is readily evident by inspecting the first principal part of any given verb. If it ends in -or, the verb is deponent. If it ends in -o, it is regular. If, in turn, you determine that the verb is deponent, then you must remember to translate it as an active verb.

There may be another clue to a verb's being deponent. If you see a verb with a passive ending that nevertheless has what seems like a direct object construed with it, there's a good chance that you have a deponent verb. True passive verbs cannot have a direct object!

Deponent verbs, like regular verbs, fall into the four conjugations (including 3rd -io), but they have only three principal parts, not four. Here are five deponents that may serve as specimens:

CONJUGATION	1st principal part = 1st pers. sing. pres. passive indicative	2nd principal part = present passive infinitive	3rd principal part = 1st person sing. perf. passive indicative
1st	conor (I try)	conari (to try)	conatus sum (I tried)
2nd	vereor (I fear)	vereri (to fear)	veritus sum (I feared)
3rd	loquor (I speak)	loqui (to speak)	locutus sum (I spoke)
3rd -io	morior (I die)	mori (to die)	mortuus sum (I died)
4th	orior (I arise)	oriri (to arise)	ortus sum (I arose)

Deponent verbs lack a fourth principal part. Why? Because, in regular verbs, the fourth principal part is the perfect passive participle. That form is part of the third principal part of a deponent verb, so to repeat it as a separate, fourth principal part is considered superfluous.

TO SUM UP...the critical thing about deponents (other than knowing your passive forms!) is being able to recognize them when (1) you look them up or (2) when you recall them from memory. Hence, when you run across this form:

profecti eramus

you should figure that, since it's a two-part verb with eramus as the second part, the form should be identified as pluperfect passive. But, how to translate? You look up the verb and discover:

proficiscor, proficisci, profectus sum: I set out

Since there are only 3 principal parts, and since the first ends in -or, it must be depenent. Therefore, you translate this particular form as though it were pluperfect active, hence:

"we had set out."

Simple, right? (Don't answer that.)