Срок подачи указан в налоговой декларации.

Срок can mean term, time limit, or deadline. In this sentence, it means deadline.

Подачи is the genitive singular form of подача, meaning submission or filing. Russian very often uses a noun + genitive noun structure where English uses either:

• a compound noun: filing deadline
• or an of phrase: deadline of submission

So срок подачи is literally deadline of filing/submission, but naturally it means filing deadline.

Because указан is the short form of a passive participle, and short forms are commonly used as part of the predicate:

• Срок подачи указан. = The filing deadline is stated/specified.

By contrast, указанный is the full form, and it usually works like an adjective directly modifying a noun:

• указанный срок подачи = the specified filing deadline

So in your sentence, Russian needs the short form because the sentence is saying something about the subject, not just attaching a descriptive adjective to it.

Because it agrees with срок, which is masculine singular.

Short-form participles in Russian agree with the noun they refer to:

• masculine singular: указан
• feminine singular: указана
• neuter singular: указано
• plural: указаны

Since срок is masculine singular, the correct form is указан.

In this sentence, указан does not mean past tense by itself.

Russian normally omits the verb to be in the present tense. So:

• Срок подачи указан... = The filing deadline is indicated/stated...

If you wanted the past tense, you would add был:

• Срок подачи был указан... = The filing deadline was indicated...

And for the future:

• Срок подачи будет указан... = The filing deadline will be indicated...

So the sentence is present in meaning, even though English needs is.

This is the prepositional case.

The preposition в often means in. When it means location or position, it usually takes the prepositional:

• в декларации = in the declaration
• в налоговой декларации = in the tax declaration / in the tax return

The dictionary form is:

• налоговая декларация

But after в in this meaning, it becomes:

• в налоговой декларации

Both words are feminine singular here, in the prepositional case.

Because -ая / -я are dictionary-form endings, but Russian words change endings depending on case.

Dictionary form:

• налоговая декларация

Prepositional singular:

• в налоговой декларации

So the endings change because the phrase is governed by в meaning in.

This is one of the most important things to get used to in Russian: the role of a word in the sentence affects its ending.

All of those can work depending on context.

In official or administrative language, указан often means something like:

• stated
• specified
• indicated
• sometimes listed

In this sentence, a natural English translation would often be:

• The filing deadline is stated in the tax return.
• The filing deadline is specified in the tax return.

So указан is a formal document-style word here, not a physical pointed at meaning.

Yes. Russian word order is much more flexible than English word order.

For example, you could also say:

• В налоговой декларации указан срок подачи.

That still means the same basic thing. The difference is mostly in emphasis:

• Срок подачи указан в налоговой декларации.
  Focuses first on the filing deadline.

• В налоговой декларации указан срок подачи.
  Focuses first on where the information appears.

Because Russian uses case endings, it can move words around more freely without changing the basic grammatical relationships.

Yes, but the nuance is a little different.

• срок подачи = the standard, compact way to say filing deadline
• срок для подачи = more literally time/period for filing

In official and bureaucratic Russian, срок подачи is very common and sounds natural here.

So while срок для подачи is possible in some contexts, срок подачи is the more idiomatic choice in this sentence.

Russian has no articles, so there is no separate word for the or a/an.

Whether something is definite or indefinite is usually understood from:

• context
• word order
• the situation
• sometimes words like этот (this) if extra clarity is needed

So Срок подачи указан в налоговой декларации can naturally mean The filing deadline is stated in the tax return, even though there is no word exactly equivalent to the.