На рассвете я иногда читаю рецензии на новые книги, а в сумерки иду в библиотеку.

These are both common time expressions, but they are formed differently in Russian, and you mostly learn them as set patterns.

• на рассвете = at dawn / at daybreak
• в сумерки = at dusk / at twilight

So the difference is not just a general rule like English at vs in. Russian often uses fixed combinations for times of day or natural phases.

Here:

• на рассвете uses на + prepositional
• в сумерки uses в + accusative

That may feel inconsistent at first, but it is normal in Russian. Many temporal expressions are partly idiomatic.

Рассвете is the prepositional singular of рассвет.

The basic noun is:

• рассвет = dawn, daybreak

After на in this expression, it becomes:

• на рассвете

So this is a set temporal phrase meaning at dawn.

Сумерки is one of those Russian nouns that is normally used only in the plural. This is called a plural-only noun.

So you usually say:

• сумерки = twilight, dusk

and not a normal singular form in everyday use.

Because of that, other forms also stay plural-looking:

• в сумерки
• в сумерках

Even though English uses a singular-looking word like twilight or dusk, Russian uses a plural noun here.

Both exist, but they mean slightly different things.

• в сумерки = at dusk, focusing on when
• в сумерках = in the twilight / in the half-darkness, often focusing more on the surrounding state or setting

So in this sentence, в сумерки is the natural choice because it means at that time of day.

Because the sentence describes a repeated or habitual action:

• иногда читаю = I sometimes read

In Russian, habitual, repeated, or ongoing actions normally use the imperfective aspect.

So:

• читать → читаю = imperfective, suitable for routine or repeated action

A perfective present form would not work here the way English does, because Russian perfective verbs do not normally have a true present meaning. Their present-looking forms usually refer to the future.

It is accusative plural, because it is the direct object of читаю.

The verb читать takes a direct object in the accusative:

• читать что? = to read what?

So:

• рецензии = accusative plural here

It looks the same as the nominative plural because рецензия is an inanimate noun.

Compare:

• nominative plural: рецензии
• accusative plural: рецензии

That is very common with inanimate nouns in Russian.

Because the noun рецензия commonly takes the pattern:

• рецензия на что? = review of / review on what?

So Russian says:

• рецензия на книгу
• рецензии на новые книги

This is a matter of government, meaning the noun naturally goes with a certain preposition and case. It is best learned as a chunk:

• рецензия на + accusative

Also, новые книги is in the accusative plural, but since книги is inanimate plural, it looks the same as nominative plural.

Because иду shows movement toward a destination, and Russian uses:

• в + accusative for motion into / toward a place

So:

• иду в библиотеку = I am going to the library

But:

• в библиотеке = in the library, inside the library

That form would be used for location, not destination. For example:

• Я читаю в библиотеке = I read in the library

So the contrast is:

• в библиотеку = to the library
• в библиотеке = in the library

That is a very good question, because both verbs are related, but they do not mean exactly the same thing.

• идти = to go on foot in one concrete direction
• ходить = to go on foot generally, repeatedly, or in multiple directions

A learner might expect:

• в сумерки хожу в библиотеку

That would emphasize a general habit: I go to the library at dusk.

But иду can be used in a more vivid, scene-like way: the speaker mentally places us in that repeated moment, as if each time, at dusk, I head off to the library.

So:

• хожу = more general habitual fact
• иду = more concrete, more immediate, more like picturing the trip itself

In a textbook-style sentence about routine, хожу might also be possible, but иду gives a slightly different feel rather than being simply wrong.

А often means more than simple and. It links two ideas while also showing a contrast or shift.

Here the sentence contrasts:

• на рассвете vs в сумерки
• reading reviews vs going to the library

So а is very natural: it signals and meanwhile / whereas / while on the other hand.

If you used и, it would sound more like just adding another action with less contrast.

Yes. Russian word order is flexible, and иногда can move depending on emphasis.

For example, all of these are possible:

• На рассвете я иногда читаю рецензии...
• Я иногда на рассвете читаю рецензии...
• Иногда на рассвете я читаю рецензии...

The version in the sentence is quite natural and neutral. It gives a smooth order:

1. time frame
2. subject
3. frequency
4. verb
5. object

So the meaning stays basically the same, but the emphasis can shift slightly.

Yes, it could be omitted, because the verb endings already show first person singular:

• читаю
• иду

So Russian could say:

• На рассвете иногда читаю рецензии на новые книги, а в сумерки иду в библиотеку.

That is grammatically fine.

But including я is also completely normal. It can make the sentence:

• clearer
• more balanced
• slightly more explicit

Russian often omits subject pronouns when they are obvious, but it does not have to.

Because а joins two separate clauses, each with its own verb:

• я иногда читаю...
• иду в библиотеку

In Russian, a comma is normally required before а in this kind of compound sentence.

So the comma is standard punctuation here, not optional style.