Als er morgen nog iets over is, nemen Anna en Sofie het mee in bakjes voor de lunch.

Here als means if.

In Dutch, als can introduce:

• a condition: if
• a time clause: when

In this sentence it is clearly conditional: If there is still something left tomorrow, Anna and Sofie will take it along...

So the idea is not that there definitely will be leftovers, but that this depends on what happens.

Because als introduces a subordinate clause, and in Dutch subordinate clauses the finite verb usually goes to the end.

So:

• main clause: Er is morgen nog iets over
• subordinate clause: Als er morgen nog iets over is

This is one of the biggest word-order differences between English and Dutch.

Er is a very common Dutch word with several uses. Here it is the existential er, like the there in English there is/there are.

Compare:

• Er is nog iets over = There is still something left

You usually get this er when:

• something is being introduced
• the subject is indefinite, like iets, iemand, een probleem, geen tijd

So er does not mean a place here. It is just part of the structure.

Over zijn means to be left over / to remain.

So:

• iets over zijn = something is left
• nog iets over zijn = there is still something left

Here:

• nog = still
• iets = something
• over is part of the expression over zijn

English speakers may want to read over as about or finished, but here it means left over.

Because Dutch main clauses follow the verb-second rule.

The first whole part of the sentence is the subordinate clause:

• Als er morgen nog iets over is

That entire clause takes the first position. Then the finite verb of the main clause must come next:

• nemen

So the order becomes:

• Als er morgen nog iets over is, nemen Anna en Sofie het mee...

If there were no fronted clause, you would normally say:

• Anna en Sofie nemen het mee...

Because meenemen is a separable verb.

Its parts are:

• mee = along / with
• nemen = take

In a main clause with a finite verb, separable verbs split:

• Anna en Sofie nemen het mee

But in infinitives or participles, they stay together:

• Ze willen het meenemen
• Ze hebben het meegenomen

So nemen ... mee is exactly what you should expect in this kind of sentence.

Het refers back to whatever is left over.

The earlier phrase is iets. Since iets is neuter and indefinite, Dutch often refers back to it with het:

• Als er nog iets over is, nemen ze het mee

So het here means it.

Even though iets means something, once you refer back to that leftover thing, Dutch naturally uses het.

With separable verbs, the separated part often comes late in the clause, but not always literally as the last word. Other parts can come after it.

Here:

• nemen Anna en Sofie het mee in bakjes voor de lunch

This is very natural. The particle mee comes after the object pronoun het, and then the sentence continues with extra information:

• in bakjes
• voor de lunch

So mee is separated from nemen, but it does not have to be the absolute final word.

Bakjes is the plural of bakje, which is a diminutive form of bak.

A bakje is a small container, tub, tray, or food box. In this sentence it most likely means small food containers.

Why the diminutive?

• Dutch uses diminutives very often in everyday speech
• they do not always sound cute; they can just sound natural and practical

So in bakjes means something like:

• in containers
• in little tubs
• in food boxes

The plural suggests that the food will be divided into containers rather than put into one single container.

That could mean:

• one container for Anna and one for Sofie
• several small portions
• just a general idea of packing food into lunch containers

Dutch often uses the plural in this kind of practical, everyday context.

Voor de lunch means for lunch in the sense of intended for lunch.

So the idea is:

• they will take the leftovers with them
• packed in containers
• to eat at lunch

It does not mean before lunch here. That would only be the meaning if the context clearly pointed to time.

So in this sentence, voor de lunch = for lunch / as lunch food.

Yes, absolutely. Iets and wat can both work here.

• iets is a little more neutral and standard
• wat is very common in everyday spoken Dutch

So both of these are natural:

• Als er morgen nog iets over is...
• Als er morgen nog wat over is...

The meaning is basically the same here.

Because Dutch normally uses a comma between a subordinate clause and the main clause when the subordinate clause comes first.

So:

• Als er morgen nog iets over is, nemen Anna en Sofie het mee...

That comma helps mark the boundary between:

• the if-clause
• the main clause

This is very standard punctuation in Dutch.

Not completely fixed, but this order is very natural.

In:

• Als er morgen nog iets over is

you get:

• er as the existential placeholder
• morgen for time
• nog for still
• iets as the indefinite subject-like element
• over
• is at the end because it is a subordinate clause

Dutch adverb order can vary somewhat depending on emphasis, but this version sounds smooth and normal. A learner should treat it as a good model.