Meet alles op en installeer de kast.

Because of a Dutch spelling rule.

• In meten, the first syllable me- is an open syllable, so a single e can represent the long vowel sound.
• In meet, that vowel is now in a closed syllable, so Dutch doubles the vowel letter to keep the same long sound.

So:

• meten → infinitive
• ik meet → present tense
• Meet! → imperative

The pronunciation stays basically the same; the spelling changes to show vowel length.

Because opmeten is a separable verb.

In Dutch dictionaries, the verb is written as one word:

• opmeten = to measure up / take measurements

But in many sentence patterns, the prefix op splits off and moves later in the clause.

So:

• infinitive: opmeten
• imperative: Meet alles op
• main clause: Ik meet alles op
• subordinate clause: ... dat ik alles opmeet

This is very common in Dutch with verbs like:

• aandoen
• opruimen
• uitzoeken
• afwassen

Meten means to measure in a general sense.

Opmeten is more specific: it often means to take measurements carefully/completely, especially when you need the dimensions for something practical.

So:

• meten = measure
• opmeten = measure up, take the measurements

In this sentence, opmeten fits well because the idea is not just casually measuring one thing, but getting all the needed measurements before the next step.

Because this is an imperative sentence, meaning a command or instruction.

In Dutch, commands usually leave out the subject:

• Meet alles op
• Installeer de kast

That is the normal way to give instructions.

You can add a subject in some situations, but then it often sounds emphatic, contrastive, or less neutral. For standard instructions, the subject is usually omitted.

By itself, it is just an instruction. In practice, it can work as a general command to one person or as an instruction in a manual or checklist.

Dutch imperatives do not always make number as obvious as English learners might expect, especially in short instruction-style sentences. Here, the sentence mainly focuses on the actions, not on who exactly is being addressed.

So the safest way to read it is:

• This is an instruction/command
• the context tells you whether it is aimed at one person, several people, or just the reader in general

Because kast is a de-word in Dutch.

Dutch nouns take either:

• de
• het

You usually have to learn this with each noun. For this word:

• de kast = the cupboard / cabinet / wardrobe

A useful related form:

• het kastje = the little cabinet / cupboard

That happens because all diminutives in Dutch take het.

It can be natural, but it depends on what you mean by kast.

• If you mean install a fitted wardrobe/cabinet in place, then installeer de kast can work.
• If you mean assemble the furniture, Dutch often prefers:
  • monteer de kast
  • zet de kast in elkaar

So if this is about furniture assembly, many speakers would find monteer de kast more idiomatic than installeer de kast.

Literally, yes — alles means everything.

But in this sentence it is usually understood more practically as:

• everything that needs measuring
• all the relevant dimensions
• all parts / all measurements

So it does not necessarily mean every single thing in the room. It means everything necessary for the task.

Meet op alles is not correct here.

With separable verbs, the normal pattern is:

• Verb + object + prefix
• Meet alles op

You can also say Meet de kast op, but that changes the meaning slightly. It would mean:

• Measure the cabinet/wardrobe itself

That is more specific than Meet alles op, which suggests taking all the necessary measurements in general.