ليس في سلة النزهة عصير فقط، بل شطائر أيضا.

ليس is the negative word used with nominal or existential-style sentences in Arabic.

Here, it negates the idea في سلة النزهة عصير فقط. So the structure is basically:

• ليس ... عصير فقط
• It is not the case that there is only juice ...

It does not mean past tense here. Even though ليس is historically verb-like in grammar, in meaning it works here like is not / there is not.

Modern Standard Arabic often expresses possession or contents by using an existence structure instead of a direct verb like English have.

So instead of saying:

• The picnic basket doesn’t have only juice

Arabic naturally says something closer to:

• In the picnic basket, there is not only juice ...

That is why you see:

• في سلة النزهة = in the picnic basket

This kind of structure is very common in Arabic.

This is a very common Arabic word order in existential sentences.

Arabic often puts the location first:

• في سلة النزهة = in the picnic basket

and then gives the thing that exists there:

• عصير
• شطائر

This sounds natural in Arabic, especially when the noun after it is indefinite.

So the order is not random. It is the normal Arabic way to say something like:

• There is juice in the basket
• literally: In the basket [there is] juice

It is an iḍāfa construction, often called a possessive or of-construction.

So:

• سلة = basket
• النزهة = the picnic / the outing

Together:

• سلة النزهة = the picnic basket
• literally: the basket of the picnic

A few important things happen in an iḍāfa:

• the first noun usually does not take ال
• the first noun also does not take tanwīn
• the second noun is in the genitive

Because of في, the first noun is also genitive here:

• في سلةِ النزهةِ

Because the sentence is talking about what items are present in the basket, not about specific already-identified items.

So:

• عصير = some juice / juice
• شطائر = sandwiches

This is very natural in Arabic existential sentences. After a phrase like في سلة النزهة, an indefinite noun often appears.

If you used العصير or الشطائر, the meaning would sound more like you are referring to specific known juice or specific known sandwiches.

This is the key pattern in the sentence.

• فقط = only
• بل = rather / but rather / but
• أيضا = also / too

So the logic is:

• ليس ... عصير فقط = not only juice
• بل شطائر أيضا = but sandwiches also

Together:

• not only juice, but also sandwiches

In Arabic, this pattern is very common:

• ليس X فقط، بل Y أيضا

You may also see related patterns such as:

• ليس فقط X، بل Y أيضا

Both are common, though the wording and emphasis can shift a little.

Because Arabic often places these words after the thing they modify.

So:

• عصير فقط = juice only
• شطائر أيضا = sandwiches too / sandwiches also

This is very natural Arabic word order.

An English speaker may expect:

• only juice
• also sandwiches

But Arabic very often prefers:

• juice only
• sandwiches also

Both are nominative in this sentence.

A fuller vocalized version would be:

• ليس في سلةِ النزهةِ عصيرٌ فقط، بل شطائرُ أيضًا

Why nominative?

• عصيرٌ is the postponed ism of ليس
• شطائرُ is coordinated with it, so it matches that case

A useful extra point:

• عصيرٌ takes tanwīn
• شطائرُ does not take tanwīn here

That is because شطائر is a diptote form, so in the nominative it takes ḍamma but no tanwīn.

In this kind of existential sentence, Arabic very often uses ليس in the default masculine singular when the noun comes later in the sentence.

So:

• ليس في ... عصير ...
• ليس في ... كتب ...
• ليس في ... مشكلة ...

This is a very common pattern.

In your sentence, that default usage fits well, and the first noun after the location is also عصير, which is masculine singular.

So even though شطائر is plural, ليس is still perfectly natural here.

The singular is:

• شطيرة = sandwich

The plural is:

• شطائر = sandwiches

This is a broken plural, not a regular sound plural.

So it does not form the plural by simply adding a regular ending like:

• ـون / ـين
• or ـات

Instead, the internal shape of the word changes:

• شطيرة → شطائر

That is very common in Arabic vocabulary.

A careful pronunciation would be approximately:

• laysa fī sallati n-nuzhati ʿaṣīrun faqaṭ, bal shaṭāʾiru ayḍan

A few pronunciation notes:

• سلة has a doubled l sound: salla-
• النزهة is pronounced with assimilation of ال to ن, so an-/n- rather than a clear al
• شطائر contains a hamza: shaṭāʾir
• أيضا is pronounced ayḍan in full case pronunciation

In normal everyday reading, many speakers will not pronounce all the case endings, but the sentence structure stays the same.