יש כורסה ישנה ליד השידה, אבל הכורסה עדיין נוחה מאוד.

יש means there is or there are.

So:

• יש כורסה ישנה... = There is an old armchair...

A very common Hebrew pattern is:

• יש + noun = there is/there are + noun

It is also used for possession in Hebrew:

• יש לי ספר = I have a book
  literally, There is to me a book

In present-tense Hebrew, the verb to be is usually omitted.

So Hebrew says:

• הכורסה נוחה
  literally: the armchair comfortable

but it means:

• The armchair is comfortable

That is completely normal Hebrew.
In past or future, Hebrew does use forms of to be, but not usually in the present.

Because the first mention is indefinite, and the second mention is definite.

• כורסה = an armchair
• הכורסה = the armchair

So the sentence works like this:

1. It introduces something new: יש כורסה ישנה... = There is an old armchair...
2. Then it refers back to that same armchair: אבל הכורסה... = but the armchair...

This is very similar to English:

• There is an old armchair... but the armchair is still very comfortable.

In Hebrew, adjectives usually come after the noun they describe.

So:

• כורסה ישנה = an old armchair
• literally: armchair old

This is the normal word order in Hebrew.

More examples:

• בית גדול = a big house
• ילדה חכמה = a smart girl
• ספר מעניין = an interesting book

Because כורסה is a feminine singular noun, and Hebrew adjectives must agree with the noun in:

• gender
• number
• and, when relevant, definiteness

So:

• כורסה is feminine singular
• therefore:
  • ישנה = feminine singular form of old
  • נוחה = feminine singular form of comfortable

Compare:

• כיסא ישן = an old chair (masculine)
• כורסה ישנה = an old armchair (feminine)

And:

• כיסא נוח = a comfortable chair
• כורסה נוחה = a comfortable armchair

Yes — in unvocalized Hebrew, ישנה can look ambiguous.

It can represent different words depending on context. But here, because it comes right after כורסה and functions as an adjective, it clearly means old.

So:

• כורסה ישנה = an old armchair

The context makes the meaning clear.

ליד means next to, beside, or by.

So:

• ליד השידה = next to the dresser / bedside table

It works like a preposition in English:

• ליד הבית = next to the house
• ליד הדלת = next to the door

So in this sentence:

• יש כורסה ישנה ליד השידה
  = There is an old armchair next to the dresser

Because it means the dresser / the bedside table, not just a dresser.

The prefix ה־ is the Hebrew definite article, equivalent to English the.

So:

• שידה = a dresser / a bedside table
• השידה = the dresser / the bedside table

Hebrew adds ה־ directly to the beginning of the noun.

עדיין means still.

So:

• הכורסה עדיין נוחה מאוד = the armchair is still very comfortable

It shows that something continues to be true, even though you might expect otherwise.

Here the idea is:

• the armchair is old,
• but it is still very comfortable.

In Hebrew, מאוד usually comes after the adjective or adverb it modifies.

So:

• נוחה מאוד = very comfortable
• literally: comfortable very

This is normal Hebrew word order.

More examples:

• גדול מאוד = very big
• יפה מאוד = very beautiful
• לאט מאוד = very slowly

Yes, absolutely.

Both are natural:

• אבל הכורסה עדיין נוחה מאוד
• אבל היא עדיין נוחה מאוד

Using הכורסה again can sound a bit clearer or slightly more explicit, especially for contrast:

• There is an old armchair... but the armchair is still very comfortable.

Using היא is also very common once the noun is already understood:

• ...but it is still very comfortable.

Because the sentence is introducing the existence of something.

• יש כורסה ישנה ליד השידה = There is an old armchair next to the dresser

Using יש is the normal way to say that something exists or is present somewhere.

If you simply said כורסה ישנה ליד השידה, it would sound incomplete in standard Hebrew, because you would be missing the normal existential structure.

So יש is important here: it sets up the meaning there is.

It has two parts joined by אבל = but:

1. יש כורסה ישנה ליד השידה
   = There is an old armchair next to the dresser

2. אבל הכורסה עדיין נוחה מאוד
   = but the armchair is still very comfortable

So the whole sentence contrasts two ideas:

• the armchair is old
• but it is still very comfortable

That contrast is exactly what אבל is doing.