"We will compose a new world anthem."
Translation:Složíme novou světovou hymnu.
Your assumption is faulty. Prefixing is just a way to turn an imperfective verb into a perfective one (e.g. skládat -> poskládat). There are many perfective verbs without a prefix, such as "skočit" (vs. skákat), "sednout (si)" (vs. sedat (si)), "říct" (vs. říkat), "hodit" (vs. házet), "vzít" (vs. brát)... they are no exceptions.
The one in this exercise is actually tricky, because it IS prefixed. The problem is that the unprefixed "ložit" doesn't mean anything anymore (and the clearly related noun "lože" is an archaic word for "bed"). And the imperfective variant "klást" (to lay down) is a bit bookish in many contexts. However, there are various prefixed perfective verbs derived from "-ložit":
- položit - to lay, to put down
- založit - to found, also to file
- naložit - to handle (when used with "s +ins."), also to load (cargo), or to pickle
- složit - to compose, to assemble, also to unload (cargo on the ground)
- rozložit - to decompose, to disassemble
- přeložit - to translate, also to move (cargo/materials) from a place to another
- přiložit - to attach, also to add solid fuel to fire
- odložit - to postpone
- vložit - to insert, to enter (data)
- vyložit - to interpret, also to unload cargo (out of a vehicle)
- předložit - to bring/put forward
- uložit - to deposit, to save (data), also to issue (a fine, a task)
- vynaložit - to expend (effort, resources)
All of these can be turned imperfective by replacing "-ložit" with "-kládat" (e.g. "pokládat"). Some of these verbs can then be turned back into perfective one by adding another prefix, e.g. "po-s-kládat" (to completely assemble piece by piece) or "do-pře-kládat" (to finish translating). And BTW, "do-pře-ložit" also works, but "po-s-ložit" doesn't. ... o.O ... And another BTW, both "poskládat" and "dopřekládat" can be made imperfective again, although this is uncommon: "poskládávat" (to repeatedly completely assemble by pieces) and "dopřekládávat" (to repeatedly finish translating).
Fun with aspects, huh? :)