Learn Danish: Du skal ringe når du hører alarmen. - You must call when you hear the alarm.

While skal looks like the English word "shall", it is a "false friend." In Danish, skal most commonly means "must" or "have to." It is used to give a command, state a rule, or express a strong obligation.

Why is it just "ringe" and not "at ringe" (to call)?

After a modal verb like skal (must), you use the basic dictionary form of the next verb without at (to). This is called the bare infinitive. Because of this rule, you just say skal ringe.

Could I use "da" instead of "når" for "when" in this sentence?

No, you must use når here. In Danish, når is used for "when" in the present and future tenses, or for things that happen repeatedly. The word da is strictly used for a single, specific event in the past (like "when the alarm went off yesterday").

Where is the word for "the" in "alarmen"?

In Danish, "the" usually isn't a separate word before the noun. Instead, it is attached directly to the end of the noun as a suffix. The base word is alarm, and because it is an "en" word, you add -en to the end to get alarmen (the alarm).