What's the accusative case for

1. magna urbs
2. rex fortis
3. navis longa
4. matres laetae
5. omnes puellae

accusative singular? or plural?

Use this chart of noun endings:

http://www.math.osu.edu/~econrad/lang/ln.html

This is an excellent website that will help with nouns & adjectives, as well as verbs:
http://www.math.osu.edu/~econrad/lang/latin.html

Let me know what you decide for each of your phrases above, and I'll check them for you.

The 3rd delension

What's the accusative case for
1. magna urbs
2. rex fortis
3. navis longa
4. matres laetae
5. omnes puellae

what's the ablative case for
1. bonus rex
2. puer fortis
3. omnes comites
4. princeps tristis
5. urbes multae

Did you read and study those websites I sent you?

What did you not understand about this?
Let me know what you decide for each of your phrases above, and I'll check them for you.

I don't know if I need to put them in singular or plural. that's what making me confused. I'm doing the 3rd declension

OK, here's what I see:

The 3rd delension
What's the accusative case for
1. magna urbs
2. rex fortis
3. navis longa
4. matres laetae (plural)
5. omnes puellae (plural)

what's the ablative case for
1. bonus rex
2. puer fortis
3. omnes comites (plural)
4. princeps tristis
5. urbes multae (plural)

The unlabeled ones are singular.

Using the chart here - http://www.math.osu.edu/~econrad/lang/ln.html - use the column labeled 3mf.

I'll do two of these for you:

accusative for magna urbs is magnam urbem.

ablative for omnes comites is omnibus comitibus.