У неділю ми гуляємо в лісі.

Because неділю is the accusative singular form of неділя.

After у / в when talking about a day as a point in time, Ukrainian commonly uses the accusative:

• у неділю = on Sunday
• у середу = on Wednesday
• у п’ятницю = on Friday

So:

• неділя = nominative, the dictionary form
• неділю = accusative, used here after у

Yes, у / в often means in, but it also has time uses, where English would say on.

So:

• у неділю literally uses у, but the natural English translation is on Sunday

This is just a normal Ukrainian pattern for days of the week.

In many cases, у and в are just two phonetic variants of the same preposition. Ukrainian often chooses between them for euphony — to make the sentence sound smoother.

In this sentence:

• У неділю sounds natural at the beginning
• гуляємо в лісі sounds smoother than гуляємо у лісі

So the choice is mostly about pronunciation flow, not a change in meaning.

Both pairs usually mean the same thing:

• у лісі / в лісі = in the forest
• у неділю / в неділю = on Sunday

But one version may sound more natural in a specific sentence.

It can depend on context.

In isolation, у неділю most directly means on Sunday. But because the verb is in the present tense and can describe a habitual action, the whole sentence can also be understood as:

• On Sunday we walk in the forest
• or more naturally in English, On Sundays we walk in the forest

If you want to make the repeated meaning very explicit, Ukrainian often uses other patterns too, such as:

• щонеділі = every Sunday
• по неділях = on Sundays

So this sentence can be either a specific-time statement or a habitual one, depending on context.

Гуляємо comes from гуляти.

Here it means something like:

• we walk
• we go for a walk
• we stroll
• we spend time outdoors walking

It is not just movement from one place to another. It usually suggests walking for pleasure, leisure, or relaxation.

So in this sentence, гуляємо is better understood as we go for a walk in the forest than simply we are going.

Because гуляти and йти are not the same.

• йдемо = we are going / we are walking somewhere, with focus on movement or direction
• гуляємо = we are walking around / taking a walk / strolling, with focus on the activity itself

Compare:

• Ми йдемо в ліс. = We are going to the forest.
• Ми гуляємо в лісі. = We are walking in the forest.

So гуляємо fits because the idea is being in the forest and walking there for pleasure.

Лісі is the locative singular form of ліс.

After в / у when it means in a place, Ukrainian often uses the locative case:

• ліс = forest
• в лісі = in the forest

So:

• в = in
• лісі = locative form required after that preposition

This is a very common pattern:

• в місті = in the city
• у школі = in school
• в парку = in the park

Yes. У лісі and в лісі both mean in the forest.

The difference is usually about what sounds smoother in the sentence. After гуляємо, speakers often prefer в лісі because it flows better.

So:

• гуляємо в лісі = very natural
• гуляємо у лісі = also understandable, but may sound a little less smooth in this particular sentence

It could be omitted.

Ukrainian often leaves out subject pronouns when the verb ending already makes the subject clear.

• гуляємо already means we walk / we are walking

So both are possible:

• Ми гуляємо в лісі.
• Гуляємо в лісі.

Including ми can add emphasis, contrast, or clarity. Without it, the sentence is still perfectly normal.

It is the present tense form of an imperfective verb.

That means it can describe:

1. an action happening now
   • We are walking in the forest
2. a habitual or regular action
   • We walk in the forest
   • We go walking in the forest

In this sentence, because of у неділю, many learners will understand it as a habitual statement, especially in English:

• On Sundays we walk in the forest

But context decides.

Ukrainian word order is fairly flexible, but different orders change the emphasis.

The neutral version is:

• У неділю ми гуляємо в лісі.

Other possible orders include:

• Ми гуляємо в лісі у неділю.
  Focuses a bit more on the action first, then gives the time.
• В лісі ми гуляємо у неділю.
  Emphasizes in the forest.
• Ми у неділю гуляємо в лісі.
  Puts some emphasis on on Sunday.

So the basic meaning stays the same, but the information focus shifts.

Usually, no.

In Ukrainian, days of the week are normally written with a lower-case letter:

• неділя
• понеділок
• вівторок

So У неділю ми гуляємо в лісі. is written correctly with lower-case неділю.