Straffen känns orättvis.

Not in standard Swedish. There is a grammar mismatch.

• straffen is most naturally the punishments: the definite plural of ett straff
• orättvis is a singular adjective form used with an en-word

Those do not agree with each other. The standard versions would be:

• Straffen känns orättvisa. = plural
• Straffet känns orättvist. = singular

So the sentence as written is probably a mistake or typo.

Straffen is the definite plural form of ett straff.

The full pattern is:

• ett straff = indefinite singular
• straffet = definite singular
• straff = indefinite plural
• straffen = definite plural

This is easy to mix up, because many learners expect -en to mean singular definite, but here it is plural definite.

In Swedish, a predicate adjective usually agrees with the noun in gender and number.

For orättvis, the main forms are:

• orättvis = singular, common gender (en-word)
• orättvist = singular, neuter (ett-word)
• orättvisa = plural

Since straff is an ett-word, singular would be:

• Straffet känns orättvist.

If the subject is plural, you need:

• Straffen känns orättvisa.

A useful extra point: in predicate position, the adjective does not change just because the noun is definite. So you say:

• Straffet är orättvist

  not

• Straffet är orättvisa

Because the verb here is kännas, not känna.

• känna = to feel, know, sense
• kännas = to feel, seem, be perceived as

In this sentence, the idea is that the punishment feels unfair, so Swedish uses kännas:

• Straffet känns orättvist.

The -s is part of the verb form. It often looks passive to learners, but with verbs like kännas, it is just the normal dictionary form and meaning.

Because känner sig is usually used when a person feels a certain way about themselves.

For example:

• Jag känner mig trött. = I feel tired.

But when something seems or feels a certain way, Swedish often uses känns:

• Det känns konstigt. = It feels strange.
• Straffet känns orättvist. = The punishment feels unfair.

So in this kind of sentence, känns is the natural choice.

Because Swedish usually puts the definite article on the end of the noun.

So:

• straffet = the punishment
• straffen = the punishments

English uses a separate word (the), but Swedish often uses a suffix instead.

If there is an adjective before a definite noun, Swedish often uses both a separate article and a suffix:

• det orättvisa straffet = the unfair punishment
• de orättvisa straffen = the unfair punishments

But in your sentence, there is no adjective before the noun, so the ending on the noun is enough.

Both can be possible, but they are not exactly the same.

• Straffet är orättvist = a direct statement: the punishment is unfair
• Straffet känns orättvist = it feels/seems unfair, often more subjective

So känns can sound a little softer or more based on impression, while är sounds more definite.

Yes. The basic order is:

• subject + verb + adjective/complement

So:

• Straffet känns orättvist.
• Straffen känns orättvisa.

This is a very normal Swedish sentence pattern.

If you move something else to the front, Swedish still keeps the finite verb in second position:

• Ibland känns straffet orättvist. = Sometimes the punishment feels unfair.

That verb-second pattern is very important in Swedish.