Ég prófa súpuna.

súpuna is the definite accusative singular of the feminine noun súpa.
• Núetímaprósent (present tense verb) takes a direct object in the accusative.
• Definite = “the soup,” so you add -na rather than the indefinite -u (which would give súpu).

You’d use the indefinite accusative form súpu:
 Ég prófa súpu.
Here súpu = “(a) soup,” indefinite object.

prófa is a regular (weak) verb meaning “to try” or “to test.” In the present tense you conjugate it like this:
 ég prófa (I try)
 þú prófar (you try)
 hann/hún prófar (he/she tries)
 við prófum (we try)
 þið prófið (you pl. try)
 þeir/þær/þau prófa (they try)

Use the progressive construction with vera að:
 Ég er að prófa súpuna.
Or if you want the more specific sense of “tasting”:
 Ég er að smakka súpuna.

• prófa = to try or test something (can be taste, try on clothes, test a method, etc.)
• smakka = to taste something (more specific to flavor)
So Ég prófa súpuna could mean “I’m trying out the soup” (maybe testing a recipe), while Ég smakka súpuna is “I’m tasting the soup” (as in sampling its flavor).

Icelandic main clauses follow the V2 rule: the finite verb must be in second position. In a simple statement with the subject first, you get Subject-Verb-Object (SVO). If you front something else—an adverb or object—the verb still stays second. For example:
 Í dag prófa ég súpuna. (Today I try the soup.)