Only the brave try their hand at prose comp...
Most of your mistakes were based on grammatical concepts or words that you may be sketchy on or have forgotten. You may want to review something if you see you are consistently having problems with it. One of the most important things in writing (or reading) Latin though is just to have a cautious eye. It is easy to assume the meaning of a sentence before you have seen all the words, and this can lead to the overlooking of subjects and objects and other things.
1. Ager bono rege imperatus erat.
Ager specifically refers to a farm or field, a plot of land. For land in the broader sense, you could use terra. Imperare is more along the lines of "control/command", for ruling land I suggest rego, regere, rexi, rectum. Imperatus erat is pluperfect passive, for perfect passive just say imperatus est. You also need the preposition a with personal agents.
2. Miles sagitta interfectus erat.
Again, interfectus est is the correct tense.
3. Puer avis lapide interfecit.
Avis must be in the accusative case here.
4. Romanus dux Hannibale fusus erat.
Fusus means "poured". I think victus or superatus would make more sense
5. Miles agricolam gladio interfecit.
6. Hoste victi eramus.
I think they mean for you to use the plural hostibus here, since it is THE enemy and not AN enemy. You also need the preposition ab because the enemy is a person. You have pluperfect victi eramus when it should simply be perfect victi sumus.
7. Moenia civibus defensa erant.
That tense again! Change erant to sunt. Cives are people, so they require ab when they are used as ablatives of agent.
8. Urbs nostra Romulo aedificata est.
9. Romani urbem moene muniverunt.
Moenia is rarely in the singular. Use munus (moerus) if you desire the singular.
10. Gallia Britannia a mare divisa est.
Good, but mare is a pure I-stem so the ablative ends in -I = a mari.
11. Altum moene castram defendit.
Don't use moene.
12. Amicibus amatus sumus et illes amamus.
Amicus is first declension, not third. Amatus must be plural. Illes should be illos.
13. Hoste non victi erimus.
Correct, but it probably means for you to use the plural hostibus and you need the ab because you are speaking of personal agents.
14. Castra longe moene defensa est.
Longus is a first/second declension adjective and the ablative ends in -O, not -E (longe is the adverb form). Don't use moene.
15. Cives urbem defenderunt.
16. Cives civibus defensi sunt.
Cities is urbes.
17. Castram cepimus.
18. Castra nobis capta erant.
A nobis capta est.
19. Pueri docent.
Pueri should be in the accusative.
20. Pueri libris docti sunt.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae