Jeg legger målebåndet tilbake i skuffen, ellers forsvinner det.

In a normal main clause (a hovedsetning) Norwegian uses SVO word order, like English: Jeg (subject) + legger (verb) + the rest.
So Jeg legger målebåndet ... is the standard, neutral order.

Å legge literally means to lay or to put (something down), and it’s commonly used for placing objects somewhere:

• å legge boka på bordet = to put the book on the table
  It fits well when you’re placing something in a location, like a drawer.

Norwegian loves compound nouns. Målebåndet = måle (measure) + bånd (band/tape) + -et (the).
So it literally means the measuring tape. Writing it as one word is normal for Norwegian compounds.

Målebåndet = the measuring tape (a specific one you mean).
Et målebånd = a measuring tape (any one / introducing it for the first time).
In a practical context like this, you usually mean a known, specific tape measure, so the definite form is natural.

Tilbake means back (to where it belongs / back again).

• Jeg legger målebåndet i skuffen = I put the tape measure in the drawer.
• Jeg legger målebåndet tilbake i skuffen = I put it back in the drawer (implying that’s its proper place).

• i skuffen = in the drawer (inside it)
• på skuffen = on the drawer (on top of it—usually not intended)
• til skuffen = to the drawer (movement toward it; not the usual way to say you place something inside)

So i is the correct preposition for an object ending up inside a drawer.

Norwegian often uses the definite form when the location is understood from context—like the drawer you normally keep it in.
i skuffen suggests a known drawer (e.g., the usual one).

• ellers = otherwise / if not
• eller = or (a choice between alternatives)

So ellers forsvinner det means otherwise, it disappears (i.e., if you don’t put it back, it’ll vanish).

Because you’re joining two main clauses: 1) Jeg legger målebåndet tilbake i skuffen
2) ellers forsvinner det
It’s common to separate them with a comma, especially when the second clause acts like a consequence/condition with ellers.

After ellers, you still have a main-clause-like structure with the verb early: forsvinner det.
Norwegian main clauses often follow the V2 rule (the finite verb tends to be in the second position). With ellers placed first, the verb comes next:
Ellers (1st) + forsvinner (2nd) + det (subject) …

Å forsvinne means to disappear / vanish. It’s not reflexive in Norwegian. You just say:

• Det forsvinner = It disappears
  No seg is needed.