De zekeraar controleert de klimmer.

Because both zekeraar and klimmer are de-words.

In Dutch, the definite article is:

• de for singular common-gender nouns
• het for singular neuter nouns
• de for all plural nouns

So here:

• de zekeraar
• de klimmer

Both are singular nouns, and both happen to take de.

Also, Dutch does not normally change the article for subject and object, so you can have de on both nouns in the same sentence.

You know mainly from word order.

In a normal Dutch main clause, the usual pattern is:

subject + finite verb + object

So in:

De zekeraar controleert de klimmer.

• De zekeraar = the subject, the person doing the action
• controleert = the verb
• de klimmer = the object, the person being checked

If you switch the nouns, the meaning changes:

De klimmer controleert de zekeraar.
= The climber checks the belayer.

Zekeraar is a climbing word. It means belayer: the person who manages the rope and keeps the climber safe.

It is related to the verb zekeren, which in a general sense means to secure, and in climbing means to belay.

The ending -aar often makes a noun for a person who does something, similar to how English can use endings like -er in some words.

So:

• zekeren = to belay / secure
• zekeraar = belayer

Because controleert is the conjugated form of the verb, while controleren is the infinitive.

The infinitive is:

• controleren = to check / to inspect

In the present tense:

• ik controleer = I check
• jij controleert = you check
• hij/zij controleert = he/she checks

Here the subject is singular:

• De zekeraar = the belayer

So Dutch uses the third-person singular form:

• controleert

Often, no. This is an important point.

In many contexts, controleren means:

• to check
• to inspect
• to verify

So in this sentence, it usually means the belayer is checking the climber, not controlling them in the sense of ruling over them.

This is a partial false friend for English speakers. Depending on context:

• controleren = to check / inspect
• onder controle hebben = to have under control
• beheersen can mean to control or master, depending on context

Klimmer comes from the verb klimmen, which means to climb.

Dutch often makes a noun for a person by adding -er to a verb, just like English often does.

So:

• klimmen = to climb
• klimmer = climber

That makes klimmer a very transparent word for English speakers.

Because modern Dutch usually does not mark the direct object with a special case ending.

Unlike languages such as German, Dutch generally relies on:

• word order
• context

So in:

De zekeraar controleert de klimmer.

there is no special object form for de klimmer. It stays the same.

That is why word order matters so much in Dutch.

A rough English-friendly pronunciation is:

duh zay-kuh-RAAR kohn-troh-LAYRT duh KLIM-er

A few notes:

• de is usually a quick unstressed sound, like duh
• zekeraar has stress on the last syllable: -raar
• controleert also has strong stress late in the word
• klimmer starts with a short i, similar to klim in English glimmer without the g

This is only an approximation, but it is enough to get started.

To say a belayer checks the climber, use een:

Een zekeraar controleert de klimmer.

To make both nouns plural:

De zekeraars controleren de klimmers.

Notice the changes:

• zekeraar → zekeraars
• klimmer → klimmers
• controleert → controleren

The verb changes because the subject is now plural.