Mi slaccio gli stivali prima di entrare in casa.

Mi is a pronoun meaning to myself / for myself here.

In Italian, when you do something to your own body, clothes, shoes, hair, and so on, Italian often uses:

• an indirect pronoun like mi, ti, si, ci, vi
• plus the definite article (il, la, i, gli, le)

So instead of saying I unlace my boots, Italian very naturally says something closer to:

• Mi slaccio gli stivali = I unlace the boots on myself / I unlace my boots

This is very common:

• Mi lavo le mani = I wash my hands
• Si mette il cappotto = He/She puts on his/her coat
• Ti togli le scarpe = You take off your shoes

Because with clothing and body-related things, Italian usually prefers:

• pronoun + definite article rather than
• possessive adjective

So mi slaccio gli stivali sounds much more natural than slaccio i miei stivali in normal speech.

Using i miei stivali is not impossible, but it adds emphasis, contrast, or clarity, for example:

• Non i tuoi: slaccio i miei stivali. = Not yours: I’m untying my boots.

In ordinary situations, Italian assumes the boots are yours from mi.

It is often treated as a reflexive-style verb in this kind of sentence, but for a learner, the most useful thing is this:

• slacciare = to unlace / unfasten something
• slacciarsi = to unlace something on oneself, or to unlace one’s own item

Compare:

• Slaccio gli stivali. = I unlace the boots.
• Mi slaccio gli stivali. = I unlace my boots.

The second version is more natural when talking about your own boots.

So yes, it behaves like many reflexive/pronominal verbs, but the key idea is that mi shows the action affects the speaker personally.

Slaccio is the first person singular present tense of slacciare.

So:

• slaccio = I unlace / I undo / I unfasten

With stivali, the most natural English idea is I unlace my boots or I undo my boots, depending on the type of boots.

Related forms:

• slacciare = to unlace / undo
• allacciare = to lace up / fasten
• slacciato = unlaced / undone

Because stivali is a masculine plural noun whose singular form is stivale, and it begins with s + consonant (st-).

Masculine nouns beginning with s + consonant, z, ps, gn, etc. use:

• singular: lo
• plural: gli

So:

• lo stivale
• gli stivali

Other examples:

• lo studente / gli studenti
• lo zaino / gli zaini

Prima di + infinitive is used when the subject is the same in both actions.

Here, the same person does both things:

• I unlace my boots
• I enter the house

So Italian uses:

• prima di entrare = before entering

This is very common:

• Mangio prima di uscire. = I eat before going out.
• Si lava le mani prima di cucinare. = He/She washes his/her hands before cooking.

If the subject changes, Italian usually uses prima che + subjunctive:

• Aspetto prima che tu entri. = I wait before you come in.

Because Italian usually leaves out subject pronouns when they are already clear from the verb ending.

Here, slaccio already tells you the subject is I.

So:

• (Io) mi slaccio gli stivali
  Both are correct, but io is often omitted.

Italian uses io only when needed for emphasis, contrast, or clarity:

• Io mi slaccio gli stivali, tu no. = I untie my boots, you don’t.

Because casa is often used without an article in many common expressions, especially when it means home or the house one is going into/living in.

So these are very natural:

• andare a casa = to go home
• essere in casa = to be at home / in the house
• entrare in casa = to go into the house / enter the house

Nella casa is possible, but it sounds more specific, like talking about a particular building:

• Entro nella casa gialla. = I enter the yellow house.

In your sentence, in casa is the normal idiomatic choice.

It can suggest either, depending on context, but in this sentence it most naturally means:

• to enter the house
• or more loosely, to go inside the house / before coming into the house

Because casa often has the sense of home, English translations may vary a little:

• I untie my boots before entering the house.
• I untie my boots before going into the house.
• I untie my boots before going inside.

All fit the Italian well.

Yes, but it means something slightly different.

• Mi slaccio gli stivali = I unlace / undo my boots
• Mi tolgo gli stivali = I take off my boots

So slacciare focuses on loosening or undoing them, while togliere focuses on removing them.

Sometimes both actions happen one after the other:

• Mi slaccio gli stivali e poi me li tolgo. = I undo my boots and then take them off.

It can do either, depending on context.

Mi slaccio gli stivali prima di entrare in casa can mean:

• a habitual action: I untie my boots before entering the house
• something happening now, in a vivid present: I’m untying my boots before going in

Italian uses the simple present more widely than English does, so you often need context to know whether it is habitual, general, or happening right now.