Prepositions
A Latin preposition is half the construction — the noun's case is the other half. ad hands you the accusative, ab hands you the ablative, and the meaning of the phrase only finishes once both pieces snap together.
The trap that catches every reader is in and sub. Both take the accusative when there's motion into something (in urbem — into the city), and the ablative when something is at rest in it (in urbe — in the city).
Same word, two cases, two completely different meanings — and the verb is what tells you which one Latin had in mind.
Learnings0 core · 1 AP claim
AP framework claims (1)— verbatim from AP CED
Each preposition demands one specific case. The case carries part of the meaning — and for in/sub, it carries ALL of it.
The case isn't a translation choice — it's locked by the preposition. Get the case wrong and you mistranslate the whole phrase.
See It In Action
— B. G. i.1.4
trans always takes the accusative, even when the verb (incolunt — "they dwell") describes rest, not motion. The case is locked by the preposition, not by the action.
— B. C. i.34.5
Both phrases use in, back-to-back in the same sentence. in urbem (acc.) carries the grain INTO the city; in urbe (abl.) places the workshops AT REST inside it. One word, two cases, two meanings.
— B. C. i.33.4
ab + abl. and in + acc. work as a from/to pair — Latin marks departure and arrival with different prepositions AND different cases. Notice ab takes the long form before a vowel (urbe); before consonants you'll often see a (a Caesare).
— B. C. i.67.6
in consilio takes a preposition (no motion, just location). But prima luce — "at first light" — takes a bare ablative with NO preposition: time-when uses the ablative alone. Don't reach for in every time you see English "in."
in + acc. → "into, onto, against" / in + abl. → "in, on, among"
in aedis (acc.) = into the house vs. in aedibus (abl.) = in the house
spatial "away from" / temporal "since" / agent "by" with passive verb
ab urbe (from the city) / ab hora tertia (since the third hour) / ab hoste occisus (slain by an enemy)
literal "down from" / figurative "about, concerning"
de caelo (down from heaven) / de bello (about the war)
time-WHEN, means/instrument, and manner often take NO preposition
prima luce (at first light), gladio (with a sword), magna celeritate (with great speed)
Same preposition, two cases. The verb tells you which one. Motion → accusative; rest → ablative.
motion INTO / TOWARD
in urbem venit
he came into the city
location IN / ON / AT
in urbe est
he is in the city
Tip: Ask: does the verb describe movement (venit, currit, mittit, fugit) or a state (est, manet, habitat)? Movement → acc. State → abl. Same rule applies to sub: sub montem (motion to under) vs. sub monte (rest at the foot of).
In milites in castra contenderunt ("the soldiers hurried into the camp"), why is castra in the accusative?
Study Tips
- •Learn prepositions in two stacks — accusative ones (ad, ante, in, per, post, propter, trāns, contrā, inter) and ablative ones (ā/ab, cum, dē, ex, prae, prō, sine). The grouping does the heavy lifting; the meanings come second.
- •Whenever you see in or sub, look at the verb first. Motion verb (it, venit, currit) → accusative; state verb (est, manet, habitat) → ablative. The case is your translation.
- •Watch for cum attached to a pronoun: mēcum, tēcum, nōbīscum — never cum mē. With nouns it stays out front (cum amīcō).