Quando ho tempo, impasto il pane la mattina e lo lascio lievitare vicino alla finestra.

Because Italian usually drops subject pronouns when they are not needed.

• ho already means I have
• impasto already means I knead / I mix
• lascio already means I leave / I let

So io would normally only be added for emphasis or contrast, for example:

• Io impasto il pane, ma lui no.
  = I knead the bread, but he doesn’t.

In your sentence, leaving out io is completely natural.

Because Italian, like English, often uses the present tense for habits or repeated actions.

So this sentence is not necessarily describing what the speaker is doing right now. It describes a usual routine:

• Quando ho tempo = whenever I have time
• impasto
• lascio

This is the normal way to talk about something you do regularly.

This is a very common question.

• Quando ho tempo means when/whenever I have time
• Se ho tempo means if I have time

So:

• Quando ho tempo suggests a habit: whenever that situation happens, I do this.
• Se ho tempo suggests a condition: maybe I’ll have time, maybe I won’t.

In this sentence, quando works well because the speaker is describing a repeated pattern.

Because Quando ho tempo is an introductory subordinate clause, and in Italian it is very common to separate that from the main clause with a comma.

So the structure is:

• Quando ho tempo, = introductory clause
• impasto il pane... = main clause

If the subordinate clause comes after the main clause, the comma is often not used:

• Impasto il pane la mattina quando ho tempo.

Because avere tempo often works as a general expression meaning to have time / to be free / to have enough time.

• Ho tempo = I have time
• Non ho tempo = I don’t have time

Using il would usually make it sound more specific:

• Ho il tempo necessario = I have the necessary time
• Non ho il tempo per farlo = I don’t have the time to do it

So here ho tempo is the normal idiomatic choice.

Here it is a verb: the first-person singular present of impastare.

• io impasto = I knead / I mix

This can confuse learners because impasto is also a noun in Italian, meaning something like dough / mixture / batter depending on context.

So:

• L’impasto è pronto. = The dough / mixture is ready.
• Impasto il pane. = I knead / mix the bread dough.

The sentence structure makes it clear that here it is a verb.

Italian uses articles more often than English.

In English, we often say things like:

• I bake bread
• I eat pasta

In Italian, it is very common to say:

• faccio il pane
• mangio la pasta

In this sentence, il pane can refer to the bread I’m making or bread as the thing being prepared. Using the article sounds natural in Italian.

Because Italian often uses the article in time expressions.

• la mattina = in the morning / in the mornings

This can refer to:

• a specific part of the day
• a habitual time

Very common alternatives are:

• al mattino
• di mattina

All three can work, though they may sound slightly different in style or emphasis.

By contrast, you often get no article in expressions like:

• domani mattina = tomorrow morning
• stamattina = this morning

Lo is a direct object pronoun referring back to il pane.

• il pane is masculine singular
• so the pronoun becomes lo = it

Instead of repeating il pane, Italian uses the pronoun:

• impasto il pane
• e lo lascio lievitare

So literally: and I leave it to rise.

Because unstressed object pronouns in Italian normally go before a conjugated verb.

So:

• lo lascio = I leave it / I let it

This is standard pronoun placement.

With verb + infinitive structures, Italian sometimes allows other placements too, but lo lascio lievitare is very natural and common.

This pattern means to let / leave something to do something.

So:

• lascio lievitare = I let it rise / I leave it to rise

The structure is:

• lascio = I leave / let
• lievitare = to rise

It is similar to English expressions like:

• I let it rise
• I leave it to rise

In cooking contexts, this is a very normal construction.

Yes. That is also correct and natural.

You may hear:

• Lascio lievitare il pane
• Lascio il pane lievitare
• Lo lascio lievitare

They are all possible, but they do not emphasize exactly the same thing.

• Lo lascio lievitare sounds smooth because il pane was already mentioned.
• Lascio lievitare il pane sounds a bit more like a full statement or instruction.
• Lascio il pane lievitare puts il pane more clearly in focus.

The version in your sentence avoids repetition.

Lievitare is the standard verb for dough, bread, pizza dough, and similar mixtures rising.

Examples:

• L’impasto deve lievitare. = The dough has to rise.
• Lascio lievitare il pane. = I let the bread rise.

So this is a very common kitchen verb, especially in baking.

Because the normal standard pattern is vicino a + noun.

So:

• vicino a la finestra becomes
• vicino alla finestra

This is just a contraction:

• a + la = alla

So vicino alla finestra means near the window.

You may hear that in everyday speech in some places, but standard Italian prefers vicino alla finestra or vicino alla porta, vicino al tavolo, etc.

So for learners, the safest choice is:

• vicino a + noun
• vicino alla finestra

Italian word order is fairly flexible, so yes, la mattina can move.

For example:

• Quando ho tempo, impasto il pane la mattina...
• La mattina, quando ho tempo, impasto il pane...
• Quando ho tempo, la mattina impasto il pane...

These versions are all possible, but they shift the emphasis a little.

The original sentence sounds natural and neutral: first the condition, then the action, with la mattina placed as the time reference for the routine.