На ужин я приготовлю суп из чечевицы, потому что он сытный.

На ужин is a very common Russian way to say for dinner.

Here, ужин is in the accusative case. Since ужин is a masculine inanimate noun, its accusative form is the same as the nominative: ужин.

Russian often uses:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner

So this is something worth learning as a set expression.

Because приготовлю is the 1st person singular future of the verb приготовить.

The key point is that приготовить is a perfective verb. In Russian, perfective verbs do not have a true present tense. Forms that look present are actually used for the future.

So:

• я приготовлю = I will cook / I will prepare

This usually suggests a completed result: the soup will get made.

This is a question of aspect.

• приготовлю = perfective, focuses on the result or completion
• буду готовить = imperfective, focuses more on the process or ongoing action

In this sentence, я приготовлю суп means something like I’ll make/prepare the soup with emphasis on the fact that it will be ready.

If you said буду готовить суп, it would sound more like I’ll be cooking soup or I’m going to cook soup, with more focus on the activity itself.

Yes, very often.

Russian often leaves out subject pronouns when the verb ending already makes the subject clear. Since приготовлю clearly means I will prepare, you could also say:

На ужин приготовлю суп из чечевицы, потому что он сытный.

Including я is still completely natural. It can make the sentence a bit clearer, more personal, or slightly more emphatic.

Чечевицы is in the genitive case because it follows из.

The preposition из often means from / out of, and it normally requires the genitive.

So:

• суп из чечевицы = soup made from lentils
• literally, something like soup out of lentils

The dictionary form is чечевица. In the genitive singular, it becomes чечевицы.

Yes, you could.

Both are natural:

• суп из чечевицы = lentil soup
• чечевичный суп = lentil soup

The version with из is very transparent for learners because it literally shows the ingredient. The adjective form чечевичный is also common, but it may feel a bit more lexicalized or dictionary-like.

So both work; this sentence just uses the ingredient + из + genitive pattern.

Он refers to суп.

Since суп is a masculine singular noun, the pronoun that replaces it is он.

So:

• суп → masculine singular
• он = it/he

In English, we say it for things. In Russian, nouns have grammatical gender, so a noun like суп is replaced by он.

Because сытный means filling, substantial, satisfying as food, while сыт means full in the sense of a person or animal who has eaten enough.

So:

• суп сытный = the soup is filling
• я сыт = I am full

That means сытный is the right word to describe food. You would not normally use сыт to describe the soup itself.

Because it agrees with суп, which is masculine singular.

Russian adjectives must agree with the noun they describe in gender, number, and case.

Here:

• суп = masculine singular
• so the adjective is сытный

Compare:

• сытный суп = a filling soup
• сытная каша = filling porridge
• сытное блюдо = a filling dish
• сытные блюда = filling dishes

Because потому что introduces a subordinate clause: because it is filling.

Russian normally puts a comma before потому что, just as English often separates a because clause.

So the sentence has two parts:

• На ужин я приготовлю суп из чечевицы
• потому что он сытный

The comma marks the boundary between the main clause and the reason clause.

No, Russian word order is fairly flexible.

The sentence as written is very natural:

На ужин я приготовлю суп из чечевицы, потому что он сытный.

But other orders are also possible, with slightly different emphasis:

• Я приготовлю на ужин суп из чечевицы, потому что он сытный.
• Суп из чечевицы я приготовлю на ужин, потому что он сытный.

The original version starts with На ужин, which sets the scene first: as for dinner / for dinner. That is a very common and natural way to structure a Russian sentence.