The sun never sets during the Arctic summer in the Far North. In response, reindeer in this region must change...

Sentence Structure

• The sun never sets
  • during the Arctic summer
  • in the Far North.

• In response,
• reindeer in this region
  • must change their sleep habits.

• Instead of resting
  • when it gets dark,
• they rest
  • when they need [?] their food.

Where [?] = digest / will digest / to digest / digesting

Understanding the Meaning

Let's start from the beginning:

The sun never sets during the Arctic summer in the Far North.

• This sets up the unique Arctic situation -
  • in summer, there's constant daylight, no darkness.

In response, reindeer in this region must change their sleep habits.

• Because there's no nighttime,
  • reindeer can't use darkness as their signal for when to rest.

Now we get to the key sentence:

• 'Instead of resting when it gets dark...'
  • This confirms what we just understood -
  • they can't rest when it gets dark because it doesn't get dark.

• '...they rest when they need ______ their food.'

This is where we have the blank. Let's look at the choices:

• digest / will digest / to digest / digesting

To see what works here, let's understand what this part means:

• The reindeer rest based on when they need to do something with their food -
  • specifically, when they need to digest it.

What do we notice about the structure here?

• The key phrase is 'they need ______ their food'
  • When the verb "need" is followed by another verb to show what action is necessary, it requires a specific form.

• We're expressing that the reindeer have a necessity -
  • they need to perform the action of digesting.

• In English, when "need" expresses this kind of necessity or requirement to do something, it takes the infinitive form:
  • need + to + verb
  • "they need to rest"
  • "they need to eat"
  • "they need to digest"

So we need to digest - the infinitive form that completes the phrase "they need to digest their food."

The complete meaning: Instead of using darkness as a sleep signal (which doesn't exist in Arctic summer), reindeer rest when their bodies signal the need to digest food.

GRAMMAR CONCEPT APPLIED

Using Infinitives After "Need"

When the verb "need" expresses necessity or requirement for someone to do something, it must be followed by an infinitive (the "to" form of the verb, called an infinitive in grammar terms):

Pattern: Subject + need + to + base verb

Examples:

• The students need to study for the exam.
  • Subject: The students
  • need + to study (infinitive)

• Plants need to absorb sunlight to grow.
  • Subject: Plants
  • need + to absorb (infinitive)

• We need to finish this project by Friday.
  • Subject: We
  • need + to finish (infinitive)

In this question:

• The reindeer rest when they need to digest their food.
  • Subject: they (the reindeer)
  • need + to digest (infinitive)
  • This expresses the reindeer's necessity to perform the action of digesting

Note: Don't confuse this with "need" + noun object ("I need food") or the special passive construction with gerunds ("The house needs painting" = "needs to be painted"). When expressing that a subject needs to perform an action, always use the infinitive form.