De portionerar hellre matresterna än att slösa med maten.

Yes. De is the subject pronoun they in written Swedish.

So in this sentence:

• De = they

A useful note for learners: in everyday speech, many Swedes pronounce de and dem as dom, but in writing the standard forms are still:

• de = subject form (they)
• dem = object form (them)

Here, De is the subject of the sentence, so de is correct.

Portionerar comes from the verb portionera, which means:

• to divide into portions
• to portion out
• to serve in measured amounts

In this sentence, it suggests managing leftovers carefully by dividing or serving them in sensible portions.

Grammar-wise:

• portionera = infinitive
• portionerar = present tense

So:

• De portionerar = They portion out / They divide into portions

Hellre means rather or preferably, and it is the normal Swedish word used when comparing two choices.

So:

• hellre ... än ... = rather ... than ...

In this sentence:

• De portionerar hellre matresterna än att slösa med maten.
• They would rather portion out the leftovers than waste the food.

You should think of hellre as the standard word for expressing preference between alternatives.

Examples:

• Jag stannar hellre hemma än går ut.
  I’d rather stay home than go out.

• Hon dricker hellre te än kaffe.
  She prefers tea rather than coffee.

This is a very common pattern in Swedish.

The structure is:

• [verb] hellre ... än att [infinitive]

It means:

• would rather [do something] than [do something else]

In your sentence:

• De portionerar hellre matresterna än att slösa med maten.

Literally:

• They portion out the leftovers rather than to waste the food

Natural English:

• They would rather portion out the leftovers than waste the food.

Why is there att?
Because after än, Swedish often uses att + infinitive when the second part is an infinitive clause:

• än att slösa
• than to waste

In everyday usage, this pattern is very common and natural.

In many contexts, Swedish speakers do use both patterns, but än att + infinitive is very common and often feels especially clear and natural.

So:

• hellre ... än att slösa med maten = very standard
• hellre ... än slösa med maten = may occur, but learners should stick with än att + infinitive because it is the safest and most widely accepted pattern

If you're learning, hellre ... än att ... is the structure to remember.

Matresterna means the leftovers or more literally the food leftovers.

It is made up of:

• mat = food
• rester = leftovers / remains
• matrester = food leftovers
• matresterna = the food leftovers / the leftovers

The ending -na is the definite plural ending here.

So:

• en matrest = a food leftover / a leftover
• matrester = leftovers
• matresterna = the leftovers

Swedish often attaches the to the end of the noun instead of using a separate word.

Because the verb slösa is commonly used with the preposition med when you mean waste something.

So the pattern is:

• slösa med något = to waste something

Examples:

• slösa med pengar = waste money
• slösa med tid = waste time
• slösa med maten = waste the food

This is just how the verb is normally constructed in Swedish.
So even though English says waste the food directly, Swedish prefers slösa med maten.

Both are possible in different contexts, but maten means the food, referring to food in a specific, understood sense.

Here the sentence is talking about food that is already present in the situation—especially the food connected with the leftovers—so the definite form sounds natural:

• maten = the food

Compare:

• slösa med mat = waste food, waste food in general
• slösa med maten = waste the food, the particular food being discussed

So the definite form makes the sentence feel more concrete and specific.

Yes, portionerar is grammatically present tense.

• portionera = infinitive
• portionerar = present tense

But Swedish often uses present tense in places where English might use:

• would rather
• prefer to
• or a more general present

Because of hellre ... än ..., the whole sentence expresses preference, so English naturally translates it as:

• They would rather portion out the leftovers than waste the food.

A more literal translation like They portion out the leftovers rather than waste the food is possible, but would rather often sounds more natural in English.

The sentence follows normal Swedish main-clause word order:

• De = subject
• portionerar = verb
• hellre = adverb
• matresterna = object
• än att slösa med maten = comparison phrase

So the pattern is roughly:

• Subject + verb + adverb + object + än att + infinitive phrase

Swedish usually keeps the finite verb in the second position in main clauses, and that is exactly what happens here:

• De portionerar ...

That is a very important Swedish word-order rule.

Historically and in some older or very formal contexts, De could be used as a polite form of you, but that is not how this sentence would normally be understood today.

In modern standard Swedish, learners should read De here as:

• they

The polite De is now uncommon and would usually be clear from context. In an ordinary sentence like this one, De is simply the pronoun they.