retasu to touhu wo ireta suupu ha, yoru de mo tabeyasui desu.

Because Japanese puts a modifying clause before the noun it modifies.

So:

レタスと豆腐を入れたスープ
= soup that has lettuce and tofu added to it

This is called a relative clause. Japanese does not need a word like that or which here.

Inside the clause レタスと豆腐を入れた, the verb is 入れる (to put in / add), and レタスと豆腐 are the things being added, so they take を.

• レタスと豆腐を入れた = (someone) added lettuce and tofu

The soup itself is the noun being modified, so it is understood as the thing they were added to:

[レタスと豆腐を入れた] スープ
= the soup that someone put lettuce and tofu into

Because 入れる and 入る are different verbs:

• 入れる = to put in / to add (transitive)
• 入る = to enter / to go in / to be contained (intransitive)

入れたスープ focuses on the act of adding the ingredients. That is very natural for food descriptions and recipe-style language.

If you used 入った, the nuance would be more like soup in which lettuce and tofu are contained / ended up in it, which is less direct here.

Here, でも means something like even.

So 夜でも食べやすい means:

• easy to eat even at night

The nuance is that nighttime is a situation where some foods might feel too heavy, but this soup is still light or manageable enough then.

Depending on context, でも can sometimes feel like also at night, but in this sentence even at night is the most natural interpretation.

は marks スープ as the topic of the sentence.

So the structure is roughly:

• As for the soup with lettuce and tofu added, it is easy to eat even at night.

It is not marking the subject in the same way English does. It is setting up the soup as what the sentence is talking about.

It is made from the verb 食べる (to eat) plus やすい (easy to do).

• 食べる → 食べ
• 食べ + やすい → 食べやすい

So 食べやすい means:

• easy to eat

This pattern is very common:

• 読みやすい = easy to read
• 使いやすい = easy to use
• 分かりやすい = easy to understand

The opposite is にくい:

• 食べにくい = hard to eat

Yes. 食べやすい is an い-adjective, and adding です makes it polite:

• 食べやすい = plain
• 食べやすいです = polite

This is completely normal.

One important point: with い-adjectives, you use です, not だ.

• correct: 食べやすいです
• not natural: 食べやすいだ

Good question. Japanese can use 食べる for soup, especially when it is treated as a dish with ingredients you eat, not just a liquid you drink.

Since this soup has lettuce and tofu, it makes sense to describe it as something you eat.

If it were a very smooth, drink-like soup, 飲みやすい (easy to drink) might sound more natural.

So here:

• 食べやすい = natural if the soup is more like a light meal with ingredients
• 飲みやすい = natural if the focus is on the liquid itself

Yes. Japanese often leaves out the subject when it is obvious or unimportant.

入れた does not say who added the lettuce and tofu. It could mean:

• the cook added them
• the speaker added them
• someone in general added them

Japanese often does this, especially in food descriptions.

No. Normal Japanese is usually written without spaces:

レタスと豆腐を入れたスープは、夜でも食べやすいです。

The spaces in your example were most likely added to help learners see the parts more clearly.