Jeg smuldrer smørret i melet, før jeg laver dejen.

Those are the definite singular forms of the nouns.

• smør → smørret
• mel → melet
• dej → dejen

In Danish, the definite article is usually added to the end of the noun, instead of being a separate word like English the. In recipe language, Danish often uses the definite form because the ingredients are understood as the specific butter, flour, and dough involved in that recipe.

That is because Danish nouns have different grammatical genders.

• smør and mel are neuter nouns, so their definite singular form usually ends in -et
• dej is a common gender noun, so its definite singular form ends in -en

So:

• et smør → smørret
• et mel → melet
• en dej → dejen

Even though English does not have this kind of noun gender, it is a basic part of Danish grammar.

Smuldrer is the present-tense form of smuldre. In this baking context, it means something like crumble or rub into crumbs.

So smuldrer smørret i melet means breaking the butter up into the flour, usually with your fingers, until the mixture becomes crumbly. In recipes, this is a very natural use of smuldre.

Danish often uses the present tense when describing actions, including recipe steps or what someone normally does. So jeg smuldrer ... før jeg laver ... is grammatically normal.

The infinitives are:

• at smuldre → smuldrer
• at lave → laver

If this were a direct instruction in a recipe, Danish would often use the imperative instead:

• Smuldr smørret i melet, før du laver dejen.

So the sentence with jeg sounds more like someone describing their own process.

In Danish, i often covers both in and into, depending on the context. Here, smuldre noget i noget is the natural expression.

So smuldrer smørret i melet means that the butter is being worked into the flour. If you wanted to strongly emphasize movement, Danish can sometimes use forms like ind i or ned i, but in this recipe-style sentence, plain i is the normal choice.

Because før introduces a subordinate clause. In Danish subordinate clauses, the normal order is subject + verb:

• før jeg laver dejen

You do not use the main-clause inversion pattern there. Compare:

• Main clause: Jeg laver dejen
• Subordinate clause: før jeg laver dejen

This is a very important Danish word-order rule.

Because the part after før is a new clause, and that clause needs its own subject. Danish does not normally leave the subject understood in this kind of sentence.

So you get:

• Jeg smuldrer smørret i melet
• før jeg laver dejen

Both clauses need a subject, and in this sentence the subject happens to be jeg in both.

Yes. If the subordinate clause comes first, the main clause that follows uses inversion.

So you would say:

• Før jeg laver dejen, smuldrer jeg smørret i melet.

Notice that the main clause becomes smuldrer jeg instead of jeg smuldrer. That happens because something else has been placed first in the sentence.

It depends on which Danish comma system is being used.

With the traditional comma, you write a comma before a subordinate clause:

• Jeg smuldrer smørret i melet, før jeg laver dejen.

With the new comma, that comma can be omitted:

• Jeg smuldrer smørret i melet før jeg laver dejen.

So the comma in your sentence is normal and correct in one common system.

The letter ø has no exact English equivalent. A useful approximation is the vowel in British English bird, but with rounded lips.

Very roughly:

• før sounds a bit like fur, but with rounded lips
• smørret contains the same vowel

This is only an approximation, because Danish vowels are quite different from English ones. If you are learning pronunciation, ø is one of the sounds worth practicing separately.

Because the word is formed from smør plus the definite ending -et:

• smør
  • -et → smørret

So the spelling keeps the r from the stem and adds the ending. This is just the normal written form; it is not a special exception.

Not usually. A recipe more often uses the imperative or a more impersonal style, for example:

• Smuldr smørret i melet.
• Smørret smuldres i melet.

Your sentence with jeg sounds more like someone explaining what they do: I crumble the butter into the flour before I make the dough. So it is perfectly grammatical, but it is a bit more personal than a standard recipe instruction.