Оперение слишком длинное.

Because it agrees with оперение, which is a neuter singular noun.

• оперение ends in -ие, which is a common neuter ending
• Adjectives must match the noun in gender, number, and case
• So the dictionary form длинный becomes длинное here

Compare:

• длинный хвост = a long tail (masculine)
• длинная шерсть = long fur/hair (feminine)
• длинное оперение = long plumage/feathering (neuter)

In Russian, the verb to be is normally omitted in the present tense.

So:

• Оперение слишком длинное. = The plumage is too long.

This is completely normal Russian.
If you wanted the past or future, then a form of быть would appear:

• Оперение было слишком длинным. = The plumage was too long.
• Оперение будет слишком длинным. = The plumage will be too long.

Слишком means too, in the sense of more than is acceptable / excessively.

So:

• слишком длинное = too long

It usually goes directly before the adjective or adverb it modifies:

• слишком большое = too big
• слишком быстро = too fast

In your sentence, слишком modifies длинное.

It is in the nominative case.

This is because оперение is the subject of the sentence:

• Оперение = the plumage / feathering
• слишком длинное = too long

In simple present-tense statements like this, the subject is usually nominative, and the adjective in the predicate matches it in gender, number, and case.

Because this sentence uses the full adjective form as part of a simple descriptive statement.

• длинное = full form
• длинным = instrumental, often used with an expressed verb like было in some structures

In the present tense with no verb, Russian commonly uses the full nominative form:

• Оперение слишком длинное.

But in past/future with быть, Russian often uses the instrumental for the predicate adjective:

• Оперение было слишком длинным.

So both are connected, but they appear in different sentence patterns.

Because длинный describes physical length, while долгий usually describes duration in time.

So:

• длинное оперение = long plumage / feathering
• долгий день = a long day
• долгая зима = a long winter

Since оперение is something physical, длинное is the correct choice.

It usually means plumage, feathering, or the covering of feathers as a whole. It is more of a collective noun than the name of one individual feather.

If you want to say a feather, that is:

• перо = feather

So:

• перо слишком длинное = the feather is too long
• оперение слишком длинное = the plumage / feathering is too long

Yes, Russian word order is fairly flexible, though the most neutral order here is:

• Оперение слишком длинное.

You could also say:

• Слишком длинное оперение.

That sounds more like too-long plumage/feathering as a noun phrase, or it may feel slightly more emphatic depending on context.

Russian word order often changes for focus, emphasis, or style, but the original sentence is the most straightforward neutral statement.

Russian does not have words like a and the. Whether something is definite or indefinite usually comes from context.

So Оперение слишком длинное could mean:

• The plumage is too long
• Its plumage is too long
• sometimes just Plumage is too long, depending on context

In real use, the situation usually makes it clear what is meant.

The stress is:

• оперЕние
• слИшком
• длИнное

So roughly:

• апирЕни-е слИшкам длИннае

A more careful approximation for English speakers would be:

• a-pye-RYE-nyee SLEESH-kam DLEE-na-ye

The exact sounds are not identical to English, but the main thing is to place the stress correctly: оперЕние слишком длИнное.