Nuntius clam ad tabernam venit et mercatori consilium dat.

Nuntius is nominative singular, so it’s the most likely candidate for the subject: “the messenger.”
In Latin, the subject is typically identified by case (nominative), not by word order. The verbs venit (comes) and dat (gives) are 3rd person singular, which matches a singular subject like nuntius.

Clam is an adverb meaning “secretly / stealthily.”
It modifies the action venit: “he comes secretly.” It is not agreeing with any noun (so it’s not an adjective).

It can, but it’s flexible. Latin often places adverbs near the word they modify, but not always.
Nuntius clam venit is a very normal arrangement: subject + adverb + verb.

Ad is a preposition that commonly takes the accusative, especially for movement toward a place: “to / toward.”
So ad tabernam means “to the shop/tavern.”
Tabernam is accusative singular because ad requires it here.

Taberna can mean a shop, stall, or sometimes a tavern/inn, depending on context.
In a sentence involving a merchant (mercator), “shop” is a very natural choice, but “tavern” is also historically possible.

Consilium is accusative singular (neuter), and with dat it naturally functions as what is being given.
A quick test: if you can translate it as “gives X”, that X is usually the accusative direct object—here consilium.

Venit is present indicative: “he comes” (or “is coming” depending on context).
Latin present can sometimes be used as a historic present (present form used for vivid past narration), but that depends on the surrounding text. In isolation, it’s safest to read it as present.

By itself, et is simply “and.”
However, since the actions are naturally sequential (you typically come somewhere before giving advice), English often supplies “and then” in interpretation. Latin doesn’t need a special word here unless the author wants to emphasize timing.