subject-verb agreement question

[AndreaS355]

Book: Sentence Correction 5th Ed.
Chapter 3: subject-verb agreement.

Problem set.

Problem No. 3:

In each of the following sentences, circle the verb, underline the subject and determine whether the subject and the verb make sense together.

3. A venomous snake designated the emblem of the rebellion by the insurgency.

My answer:
-designated is the verb
-the emblem is the subject

My answer is wrong, in the solution's part it is stated that the verb "to be" should be added to this sentence because as it is, is a fragment with "designated" as a past participle.

I am a non-native english speaker, spanish is my mother tongue. My question is, how do I identify in a sentence, that a verb ending in -ed is a past participle and not the actual or main verb like in the case shown above? What the phrase above is different from a phrase like:

I studied for the GMAT yesterday

or

He behaved in a strange way

[RonPurewal]

this is not some complicated thing; it's just a combination of context and everyday common sense.

if "__ed" is something that the noun DOES, then it's a verb.

if "__ed is something done TO the noun, or a general DESCRIPTION of the noun, then it's a modifier.


e.g.,

Pilots exhausted from long flights can sleep in the lounge.
--> "exhausted" is a description. it's not something the pilots did (i.e., the pilots did not "exhaust" anything).
--> so it's a modifier, not a verb.

Pilots exhausted all the liquor at the party, so none was left for the other crew members.
--> here, "exhausted" is something the pilots did. (they "exhausted"--in other words, consumed/used up--all the alcohol.)
--> so it's a verb.

this is a huge difference, so it shouldn't be problematic.

[RonPurewal]

as for the case above:
"designate" is an action that can only be performed by people or animals. (i can designate a certain section of my home as a home office. my dog can designate an area as his territory by peeing on it.)

so, it can't be a verb here.