Se avessi i sandali giusti, mi metterei a camminare lungo il fiume anche stasera.

Because this sentence expresses a hypothetical, unreal, or unlikely situation in the present.

Italian commonly uses this pattern:

• se + imperfetto congiuntivo
• conditional present in the main clause

So:

• Se avessi i sandali giusti = If I had the right sandals
• mi metterei a camminare = I would start walking

If you said se ho, that would sound like a real, possible condition:

• Se ho i sandali giusti, mi metto a camminare
  = If I have the right sandals, I start / I’ll start walking

So avessi is used because the speaker is imagining a situation, not stating a real one.

Avessi is the imperfect subjunctive of avere.

The conjugation is:

• io avessi
• tu avessi
• lui/lei avesse
• noi avessimo
• voi aveste
• loro avessero

In this sentence, avessi means if I had in a hypothetical sense.

English does not always clearly show the subjunctive here, but Italian does.

Metterei is the present conditional of mettere.

Here, it appears in the reflexive expression:

• mi metterei a camminare

This means:

• I would start walking
• literally, something like I would put myself to walking

In Italian hypothetical sentences, the main clause often uses the conditional:

• Se avessi..., mi metterei...
• If I had..., I would start...

So metterei matches the hypothetical structure started by se avessi.

Mettersi a + infinitive is a common Italian expression meaning:

• to start doing something
• to begin doing something

So:

• mi metterei a camminare = I would start walking

Breakdown:

• mi = myself
• metterei = I would put myself
• a camminare = to walking / walking

It is more idiomatic than literal. You should learn mettersi a + infinitive as a set pattern.

Examples:

• Mi metto a studiare. = I start studying.
• Si è messo a ridere. = He started laughing.
• Ci metteremmo a correre. = We would start running.

Because the verb here is mettersi, the reflexive form of mettere.

• mettere = to put
• mettersi a + infinitive = to set oneself to, to start

So the reflexive pronoun changes with the subject:

• mi metto = I start
• ti metti = you start
• si mette = he/she starts
• ci mettiamo = we start
• vi mettete = you all start
• si mettono = they start

In your sentence, the subject is I, so the pronoun is mi:

• mi metterei

Italian tends to use the definite article more often than English.

Here:

• i sandali giusti = the right sandals

The article i is needed because sandali is:

• masculine plural
• and the adjective giusti agrees with it

So:

• il sandalo giusto = the right sandal
• i sandali giusti = the right sandals

Using the article here sounds natural in Italian. Omitting it would generally not work.

Here giusti means something like:

• the right
• the proper
• the suitable

So i sandali giusti means the sandals that are appropriate for walking along the river.

It does not necessarily mean morally right or factually correct. It is more about suitability.

Depending on context, giusto can mean:

• correct → la risposta giusta = the right answer
• fair → non è giusto = it’s not fair
• appropriate/suitable → il momento giusto = the right moment

In this sentence, it clearly means suitable.

Camminare specifically means to walk.

So:

• mettermi a camminare = to start walking

If you used andare, the meaning would be more general:

• andare lungo il fiume could mean to go along the river
• it does not focus specifically on the action of walking

Andare a camminare means to go walking, which is also possible, but it is a slightly different structure.

Your sentence emphasizes the beginning of the activity itself:

• mi metterei a camminare = I’d start walking

Lungo il fiume most directly means:

• along the river

It suggests movement following the river’s course.

Compare:

• lungo il fiume = along the river
• vicino al fiume = near the river
• sul fiume = on the river
• al fiume = at the river / to the river

So in this sentence, the speaker imagines walking alongside the river, probably following its edge or path.

Anche stasera means:

• tonight as well
• even tonight
• tonight too

It suggests that tonight is included in addition to some other time or expectation.

Depending on context, it may imply:

• the speaker has done this before,
• or the speaker would be willing to do it even tonight, despite circumstances.

Breakdown:

• anche = also, too, even
• stasera = this evening / tonight

Because stasera is a fixed adverbial form meaning:

• this evening
• tonight

It comes historically from questa sera, but in modern Italian stasera is very common and completely standard.

You may see both:

• stasera
• questa sera

Both are correct.
Stasera is often a bit more compact and conversational.

The same happens with:

• stamattina = this morning
• stanotte = tonight / during the night
• stavolta = this time

Yes, you could say:

• Se io avessi i sandali giusti, mi metterei...

But Italian usually omits subject pronouns when they are not needed, because the verb ending already shows the subject:

• avessi → first person singular
• metterei → first person singular

So omitting io is more natural unless you want emphasis or contrast.

For example:

• Se io avessi i sandali giusti, mi metterei a camminare, non lui. = If I had the right sandals, I’d start walking, not him.

Without that emphasis, leaving out io is normal.

No. In standard Italian, Se avrei... is considered incorrect in this kind of sentence.

For hypothetical present situations, Italian uses:

• Se + imperfect subjunctive
• conditional in the result clause

So the correct form is:

• Se avessi i sandali giusti, mi metterei...

Not:

• Se avrei i sandali giusti...

This is one of the most important patterns to remember in Italian conditionals.

Yes. This sentence is a textbook example of the present unreal conditional in Italian.

Pattern:

• Se + imperfetto congiuntivo, conditional present

Example from your sentence:

• Se avessi i sandali giusti, mi metterei a camminare lungo il fiume anche stasera.

Other examples:

• Se fossi libero, verrei con te.
  = If I were free, I would come with you.

• Se avessimo più tempo, finiremmo oggi.
  = If we had more time, we would finish today.

• Se potessi, lo farei.
  = If I could, I would do it.

This is a very useful structure and worth learning as a whole.

The word order is quite natural and neutral.

Structure:

• Se avessi i sandali giusti
  = conditional clause
• mi metterei a camminare lungo il fiume anche stasera
  = main clause

Within the second clause:

• mi metterei a camminare = I would start walking
• lungo il fiume = along the river
• anche stasera = even tonight / tonight too

You could move some parts for emphasis, for example:

• Se avessi i sandali giusti, anche stasera mi metterei a camminare lungo il fiume.
• Se avessi i sandali giusti, mi metterei anche stasera a camminare lungo il fiume.

But the original version sounds smooth and idiomatic.