Я читаю книгу на березі озера.

Because книгу is the accusative singular form of книга.

In this sentence, книгу is the direct object of читаю — it is the thing being read. Ukrainian marks that with case:

• книга = nominative, the basic dictionary form
• книгу = accusative singular

For many feminine nouns ending in -а, the accusative singular ends in -у.

Читаю is the 1st person singular present tense form of читати.

So it tells you:

• I am the subject
• the action is in the present
• the verb is imperfective

Because Ukrainian verbs show the person clearly, читаю already means I read / I am reading.

It can mean either one.

Ukrainian does not usually separate these two the way English does. The present tense of an imperfective verb can cover both:

• I read
• I am reading

Context tells you which is meant. In a sentence like this, many learners would naturally understand it as I am reading because it describes an action happening in a place.

No. You can often leave it out.

Because читаю already means I read / I am reading, the sentence could also be:

Читаю книгу на березі озера.

Including я is still completely normal. It can make the subject more explicit, or add a little emphasis, especially if you are contrasting it with someone else.

Because this sentence describes a location, not movement.

After на, Ukrainian uses different cases depending on meaning:

• на березі = on / at the shore → location
• на берег = onto the shore → direction/movement

So:

• Я читаю книгу на березі озера. = I am located there.
• Я йду на берег озера. = I am going to the shore.

Also notice that берег changes to березі in the locative. This is a normal stem change in Ukrainian.

This happens because березі is the locative singular form of берег.

Two things change:

1. the ending changes to show the locative case
2. the consonant г changes to з

So:

• берег = shore, bank
• на березі = on/at the shore

This kind of consonant change is common in Ukrainian inflection.

Because озера is the genitive singular form of озеро.

The phrase берег озера means the shore of the lake. In Ukrainian, when one noun depends on another like this, the second noun often goes into the genitive:

• озеро = lake
• берег озера = shore of the lake
• на березі озера = on the shore of the lake

So озера is not controlled by на directly. It depends on березі / берег.

Here на takes the locative case because it shows where something is happening.

So:

• на березі = at/on the shore

But на does not always take the same case. It can also take the accusative when it shows motion toward something:

• на берег = onto the shore

This is a very common Ukrainian pattern with prepositions of place and movement.

Yes. Ukrainian word order is more flexible than English because cases show the grammatical roles.

The neutral order here is:

Я читаю книгу на березі озера.

But you could also say:

• На березі озера я читаю книгу.
• Книгу я читаю на березі озера.

These versions are still grammatical, but the focus changes:

• starting with На березі озера emphasizes the place
• starting with Книгу emphasizes the object

So word order in Ukrainian often affects emphasis more than basic meaning.

You usually know from context, because Ukrainian has no articles like a or the.

So книгу can mean:

• a book
• the book

The surrounding situation tells you which one is intended.

This is very normal in Ukrainian, and learners gradually get used to relying on context instead of articles.

Yes, but the meaning changes a little.

• на березі озера = on/at the shore of the lake
• біля озера = near the lake / by the lake

So на березі озера is more specific: you are actually at the shoreline.
біля озера is broader: you are somewhere near the lake.

Not always literally.

In this sentence, на is part of the natural expression на березі, which means something like on the shore or at the shore. English and Ukrainian prepositions do not match perfectly word-for-word.

So it is better to learn:

• на березі as a natural location phrase

rather than trying to force на to always equal exactly one English preposition.