Cuando un lado ya está crujiente, mi padre da la vuelta al filete.

Because the sentence describes a general or repeated action: Whenever one side is already crispy, my father turns the steak over.

With cuando, Spanish uses the indicative when talking about habits, routines, or things that regularly happen.

• Cuando un lado ya está crujiente, mi padre da la vuelta al filete.
  = this happens regularly

If you were talking about a future one-time event, Spanish would often use the subjunctive:

• Cuando un lado esté crujiente, mi padre le dará la vuelta al filete.

Ya means already. It shows that the side has reached that stage.

So ya está crujiente means is already crispy, not just is crispy.

It often adds the idea of now it has become this or it has reached that point.

Spanish uses estar here because crujiente is a state or result of cooking, not a permanent characteristic.

• está crujiente = it is crispy now, in its current condition
• es crujiente = it is crispy by nature or as a general characteristic

Food being crispy during cooking is normally treated as a temporary condition, so estar is the natural choice.

Because crujiente is an adjective ending in -e, and many Spanish adjectives of that type use the same form for masculine and feminine singular.

So you get:

• el filete crujiente
• la parte crujiente

Only the plural changes:

• los lados crujientes
• las partes crujientes

Dar la vuelta a algo is a very common Spanish expression meaning to turn something over or flip something.

So:

• dar la vuelta al filete = turn the steak over

This is an idiomatic expression. Spanish often uses this structure instead of a single verb like flip.

You may also hear other verbs in some regions, but dar la vuelta a is very natural and common.

Al is the contraction of a + el.

• a + el = al

So:

• dar la vuelta al filete
• literally: give the turn to the steak
• natural meaning: turn the steak over

This contraction happens whenever a is followed by el.

Because the expression is dar la vuelta a algo. The a is part of the structure.

So the pattern is:

• dar la vuelta a la tortilla
• dar la vuelta al filete
• dar la vuelta al pescado

You cannot normally drop that a in this expression.

Because it refers to a specific steak, the one being cooked.

In context, both speaker and listener know which steak is meant, so Spanish naturally uses the definite article:

• al filete = the steak

If it were introducing a steak for the first time in a more general way, un filete could be possible, but here el filete sounds more natural.

Spanish often omits subject pronouns, but it can still use a full noun subject like mi padre when the speaker wants to identify who does the action.

• Cuando un lado ya está crujiente, da la vuelta al filete.
  = he/she turns the steak over, depending on context

• Cuando un lado ya está crujiente, mi padre da la vuelta al filete.
  = clearly tells you my father is the one doing it

So mi padre is there for clarity and information, not because Spanish always needs the subject stated.

Because the sentence begins with a time clause:

• Cuando un lado ya está crujiente, ...

In Spanish, when this kind of subordinate clause comes first, it is very normal to separate it from the main clause with a comma.

You could also reverse the order:

• Mi padre da la vuelta al filete cuando un lado ya está crujiente.

That version usually does not need a comma.

In Spain, filete usually means a thin slice or cut of meat, often beef, pork, or similar meat cooked in a pan.

Depending on context, English translations might be:

• steak
• cutlet
• fillet in some contexts, though that can be misleading if you are thinking of fish

In this sentence, steak is a very natural translation.