Small translation help needed

[JamieConnors]

I'm slowly working my way through this book on learning Latin that I was given by a friend and I just wanted some help on a some small practise exercises that were bugging me. I've only been doing this for about 2 weeks so bear with me. Okay so I'm on second declension nouns right now and there are a couple of things I'm not sure about. The finished answer for one question is "Boni sunt filii amicorum" but I can't figure out what this means. I'm thinking it's "The friends' son is good" but "filii" is only singular so should "est" replace "sunt"? There's another question asking me to translate "Castra sunt in agro" and the answer says "The camp is in the field" however, again, shouldn't "sunt" be "est" if it uses the word "is" in the translation. I'm thinking that "castra" should be plural in order to use "sunt". This could just be an error in the book itself but it seems unlikely. The book in question doesn't look to be the most popular Latin-learning book but I'm finding it helpful so far. It's just called "Latin" and is part of a "Made Simple Books" series. I'd appreciate any help.

[bedwere]

Since you studied the second declension, you should know how to decline filius. Write down all the cases, singular and plural. What can filii mean?

Although castra/castrorum is indeed plural and agrees with sunt, it means camp (singular).

[JamieConnors]

Hmm, okay. Well filii is the only other noun in the sentence so it must be of the friends but it's just the sunt in that sentence that confuses me seeing as that can only mean 'are' and not 'is'. If there is only one son why would it use 'are good'. And with castra, does that mean that even in the plural it still means one? So like "Castrae sunt" can mean "There is a camp"? Sorry I'm trying my best to understand this.

[Sceptra Tenens]

Castra sunt in agro and boni sunt filii amicorum are two very different cases.

For the castra one:

Castrae does not exist. The singular form of castra is castrum, but then it doesn't mean "camp" anymore.

There are plenty of times when sunt should be translated as "is", and when "are" should be translated as est:

Hae litterae sunt - "This is a letter (epistle)"
Hae scopae sunt - "This is a broom"
Hae aedes sunt - "This is a house"
Haec castra sunt - "This is a camp".

"These are glasses (spectacles)" - Hoc est specillum
"These are pants" - Haec est braca (can also be hae sunt bracae - it's a rare word either way)
"These are scales (for weighing)" - Haec est libra.

This is just a matter of the Romans seeing something as plural (a camp is seen as a collection of tents, not a single unit) which we see as singular, and vice versa.

For the filii one:

As a mental exercise, what would filii be if it were plural?

[JamieConnors]

Ohhh, well the book I'm reading can't be explaining things well enough because I thought castra was 1st declension; it never showed me castrum as the singular. Okay I think I get that now, I thought it was a-declension the whole time. This book I've got never said sunt can be used like you say either. It might in a later chapter, I'm not sure.

Filii would be filiorum as a plural wouldn't it; it's genitive. If sunt can mean is, then would the sentence say The friends' son is good? This sunt thing wasn't explained like you said to me which is probably why it's confusing me.

[Sceptra Tenens]

Here's a tip - filii is nominative.

[JamieConnors]

Yeah I finally figured it out last night, feel like such a idiot.