Poena severa interdum iniqua videtur.

Because poena is the subject of videtur. The sentence is about the punishment: it is the thing that seems unfair.

• poena = nominative singular, punishment / penalty
• videtur = seems

So poena is nominative because it is the grammatical subject.

Because severa agrees with poena.

Since poena is:

• feminine
• singular
• nominative

the adjective describing it must also be:

• feminine
• singular
• nominative

So:

• poena severa = a harsh/severe punishment

If it were severam, that would be accusative singular, which would not match the subject here.

They do two different jobs.

• severa is an attributive adjective: it directly describes poena
  • poena severa = a harsh punishment
• iniqua is a predicate adjective with videtur
  • iniqua videtur = seems unfair

So the structure is basically:

• [poena severa] = the harsh punishment
• [interdum iniqua videtur] = sometimes seems unfair

That distinction is very common in Latin.

Because with videtur (seems), the adjective describing the subject is usually in the nominative as well.

In English we say:

• The punishment seems unfair

In Latin, unfair is not treated like a direct object. It is a predicate adjective linked to the subject, so it agrees with poena:

• poena = feminine singular nominative
• iniqua = feminine singular nominative

So iniqua matches poena, not because it is right next to it, but because it refers to it.

Because videtur here means seems.

This comes from videor, videri, which is literally related to to be seen, but in Latin it commonly means to seem.

So:

• videt = he/she/it sees
• videtur = he/she/it is seen or, very often, he/she/it seems

In this sentence:

• poena ... videtur = the punishment seems ...

Since poena is singular, the verb is also singular: videtur.

Interdum means sometimes or at times, and it modifies the whole verbal idea iniqua videtur.

So the sense is:

• A harsh punishment seems unfair sometimes
• or more naturally: A harsh punishment sometimes seems unfair

It is an adverb, so it does not agree with any noun or adjective.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships through endings.

English strongly depends on order:

• The harsh punishment sometimes seems unfair

Latin can move words around more freely:

• Poena severa interdum iniqua videtur

This order is perfectly natural. It keeps:

• the subject phrase first: poena severa
• the adverb in the middle: interdum
• the predicate adjective near the verb: iniqua videtur

Latin often places the verb at or near the end, though not always.

You know from both meaning and sentence structure.

Both adjectives agree with poena, but they function differently:

• severa is attached directly to the noun: poena severa
• iniqua goes with videtur: iniqua videtur

A learner should look for the verb:

• If an adjective is used with a linking verb like est, videtur, fit, and so on, it is often a predicate adjective.
• If an adjective sits as part of the noun phrase, it is often attributive.

So here the most natural reading is:

• poena severa = the harsh punishment
• iniqua videtur = seems unfair

No. Classical Latin has no articles like English a/an/the.

So poena severa can mean, depending on context:

• a harsh punishment
• the harsh punishment
• simply harsh punishment

English has to choose an article, but Latin does not.

Yes. Iniquus, -a, -um can mean things like:

• unfair
• unjust
• inequitable
• sometimes hostile or unfavorable, depending on context

In this sentence, unfair or unjust is the most natural meaning. A learner should remember that Latin adjectives often cover a range of English words.

Often, yes. In Latin, placing a predicate adjective close to its verb can make the statement feel neat and pointed:

• interdum iniqua videtur = sometimes seems unfair

The sentence builds toward the judgment iniqua and then ends with the verb videtur. That is a very natural Latin way to present the idea.

It does not necessarily mean heavy emphasis, but it does give the sentence a clean rhetorical shape.