В этом гнезде сидел маленький птенец, который ещё не умел летать.

Because this phrase means in this nest, and Russian uses the prepositional case after в when talking about location.

• в = in
• этом = prepositional singular of этот = this
• гнезде = prepositional singular of гнездо = nest

So это гнездо becomes в этом гнезде when you say in this nest.

Russian word order is much more flexible than English word order. Starting with В этом гнезде puts the location first, as the setting or background: In this nest...

Then the new information comes later: сидел маленький птенец = there was / sat a little chick.

A more neutral order like Маленький птенец сидел в этом гнезде is also possible, but it shifts the emphasis slightly onto the chick rather than the nest.

Russian often prefers a more specific verb where English might use there was. Here сидел literally means was sitting, and that sounds natural for a bird/chick in a nest.

So Russian is not just saying that the chick existed there; it describes its position or state more vividly.

Also, сидел is:

• past tense
• singular
• masculine

It is masculine because it agrees with птенец, which is a masculine noun.

You can tell from the case, not just the position.

• маленький is nominative masculine singular
• птенец is nominative singular

That shows this is the subject of the sentence. In Russian, subjects do not always come before the verb. Word order can change for emphasis, but the case endings still show the grammar.

Russian has no articles, so there is no direct equivalent of English a or the.

Whether you translate it as a little chick or the little chick depends on context. In this sentence, English usually says a little chick because the chick is being introduced as new information.

Because который introduces a relative clause: который ещё не умел летать.

In Russian, relative clauses introduced by words like который are normally separated by commas. So the comma is required here.

Который means who / which / that in relative clauses.

Here it refers back to птенец. Since птенец is:

• masculine
• singular

and который is the subject inside the relative clause, it appears in the nominative masculine singular form: который.

So literally, this part is a little chick, who/that...

Here ещё with negation means yet or still in the sense of not yet.

So:

• ещё не умел летать = did not yet know how to fly / could not fly yet

On its own, ещё can mean things like still, more, or else, but with не in this sentence it clearly gives the idea of not yet.

Уметь means to know how to do something or to be able to do something because you have learned the skill.

So не умел летать means the chick didn’t know how to fly yet or wasn’t able to fly yet because it had not learned.

This is slightly different from не мог летать:

• не умел летать = lacked the skill/ability
• не мог летать = could not, perhaps because of circumstances or physical limitation

For a young chick, не умел летать is very natural.

Because уметь is usually followed by an infinitive expressing a general ability, and for that Russian normally uses the imperfective verb.

Here:

• летать = to fly in general / to be able to fly
• лететь = to be flying in one direction, in a specific instance
• полететь = to start flying / to fly off

So не умел летать means didn’t know how to fly in general, which is exactly what you want for a chick that has not learned yet.