יש קן קטן על הענף הגבוה, ולכן אנחנו לא פותחות את החלון מהר.

יש is the Hebrew existential word meaning there is / there are.

So:

• יש קן קטן = There is a small nest

Hebrew often uses יש where English uses there is/there are. It does not change for singular vs. plural:

• יש קן = there is a nest
• יש קנים = there are nests

In Hebrew, adjectives usually come after the noun.

So:

• קן קטן = a small nest
• literally: nest small

This is the normal order in Hebrew:

• בית גדול = a big house
• ילדה נחמדה = a nice girl

Also, קטן matches קן in gender and number:

• קן is masculine singular
• so the adjective is also masculine singular: קטן

Because in Hebrew, when a noun is definite (the branch), its adjective must also be definite.

So:

• ענף גבוה = a high branch
• הענף הגבוה = the high branch

This is a very important Hebrew pattern:

• noun + adjective
• and if the noun has the, the adjective also gets the

Here:

• הענף = the branch
• הגבוה = the high

So על הענף הגבוה means on the high branch.

Because it has to agree with ענף.

Hebrew adjectives agree with the noun in:

• gender
• number
• definiteness

Here:

• ענף is masculine singular
• so the adjective is masculine singular too: גבוה

If the noun were feminine, the adjective would look different. For example:

• הציפור הגבוהה would be wrong semantically for bird, but grammatically it shows the feminine ending
• a more natural example: החומה הגבוהה = the high wall

ולכן means and therefore, and so, or simply therefore/so in context.

It is made of:

• ו = and
• לכן = therefore / so

So the sentence structure is:

• There is a small nest on the high branch
• therefore / so we do not open the window quickly

It connects the first idea to the consequence in the second part.

Because אנחנו itself does not show gender, but the verb does.

Here the verb is פותחות, which is the feminine plural form.

So:

• אנחנו פותחות = we (all female) open / are opening
• אנחנו פותחים = we (male or mixed group) open / are opening

That means the speakers are understood as female in this sentence.

In the present tense, Hebrew usually makes a sentence negative by putting לא before the verb.

So:

• פותחות = open / are opening
• לא פותחות = do not open / are not opening

This is very common:

• אני כותב = I write / am writing
• אני לא כותב = I do not write / am not writing

Here:

• אנחנו לא פותחות = we do not open

את here is the direct object marker. It usually appears before a definite direct object.

So:

• החלון = the window
• את החלון = marks the window as the direct object of פותחות

It is not translated into English.

Compare:

• אני פותחת חלון = I open a window
  (no את, because it is indefinite)
• אני פותחת את החלון = I open the window
  (את appears because it is definite)

This is one of the most important grammar points in Hebrew.

מהר means quickly or fast.

In Hebrew, adverbs like מהר often come after the verb phrase, so this word order is very natural:

• פותחות את החלון מהר = open the window quickly

Hebrew word order is somewhat flexible, but this placement sounds normal and straightforward.

Also, מהר modifies the action of opening, not the window itself.

It can mean either one, depending on context.

Hebrew present tense often covers both:

• a simple present meaning: open
• a present progressive meaning: are opening

So:

• אנחנו לא פותחות את החלון מהר can mean
  • we do not open the window quickly
  • or we are not opening the window quickly

In many everyday sentences, English chooses one of these more clearly than Hebrew does.

Because in present-tense Hebrew, the verb to be is usually not expressed the way it is in English.

Instead, Hebrew often uses:

• יש for there is / there are
• and simple noun/adjective structures without a present-tense is/are

So Hebrew says:

• יש קן קטן = There is a small nest

It does not need a separate word for is here. This is completely normal in Hebrew.