The prepostition “ad” can mean “to” or “toward”. When using “ad”, the word it refers to will be in the accusative form.

Ad silvam ambulō.  (silva = forest)

The answer to the Latin phrase above is, “I am walking to the forest”. Silva ends in -am because it’s in the accusative form.

Try translating these sentences :

(1) Nautae ad actam numquam natant. (acta = seashore. natāre = to swim)

(2)  Ad īnsulam saepe nāvigās. (īnsula = island. nāvigāre = to sail)

(3) Et aquam et cibum ad casās semper portāmus. (casa = house. cibus = food)

(4) Ego ad castrum nōn volō. (castrum = castle/fort. volāre = to fly)

Here are the answers :

(1) The sailors never swim to the seashore.

(2) You often sail to the island.

(3) We always carry both water and food to the houses. *Aquam, cibum, casās are all in the accusative. There are three ways to figure out which noun modifies to “ad”. First, “et” precedes aquam and cibum, which gives you an idea of how the author wants the words to be grouped. Second, “ad” is placed right before casās, which gives an indication that the author wants to group the two together. Third, the context of the sentence would not make much sense if “ad” were to refer to aquam or cibum. It doesn’t make a lot of sense when the sentence reads, “We always carry the houses both to the water and the food”.

(4) I am not flying to the fort/castle. *Castrum ends in -um in the singular nominative and the singular accusative. This is because castrum is a second declension noun that is a neuter noun. That’s why context here is important. Castrum would make more sense in the accusative singular than the nominative singular.