Learn Hardcore Dutch: Ik merk dat het water koud is. - I notice that the water is cold.

dat introduces a subordinate clause (just like English “that”). It links the main clause (Ik merk) with the information being noticed (het water koud is).

Can you omit dat in this sentence?

In formal and written Dutch you should keep dat. In very casual speech some people drop it (“Ik merk het water koud is”), but that’s non‑standard and can sound incomplete.

Why does the verb is appear at the end of the clause?

After a conjunction like dat, Dutch uses SOV word order in subordinate clauses. So you get …dat het water koud is, with the finite verb at the end.

Why is the adjective koud unchanged instead of koude?

When an adjective follows a form of zijn (to be) in predicative position, it stays in its base form (koud). You’d use koude only attributively (e.g. het koude water).

Why is the article het used with water instead of de?

Dutch nouns are either common-gender (take de) or neuter-gender (take het). Water is a neuter noun, so it uses het.

What tense is merk, and how would you say “I noticed” in the past?

merk is present tense, first person singular of merken. To express past noticing you’d say:
Ik heb gemerkt dat het water koud was.

Could I use voelen instead of merken to say “I feel that the water is cold”?

Yes: Ik voel dat het water koud is. But voelen stresses a physical sensation, whereas merken is more general (“to notice” by any means).

How do you say “the water feels cold” with more emphasis on the tactile feeling?

Use aanvoelen:
Ik merk dat het water koud aanvoelt.
This literally means “I notice that the water feels cold.”

How are merk and water pronounced?

merk: [mɛrk] with a short e (like in bed) and a clear final k.
water: [ˈʋaːtər], where w is softer than in English, aa is long, and the r can be trilled or guttural depending on dialect.