Если оставить хотя бы тридцать сантиметров у двери, проход будет удобнее.

Russian often uses если + infinitive in general statements, advice, instructions, and impersonal conditions.

So Если оставить... means something like:

• If you leave...
• If one leaves...
• If we leave...

The subject is not named because it is general or obvious from context.

A version with an explicit subject is also possible:

• Если вы оставите хотя бы тридцать сантиметров у двери, проход будет удобнее.

That sounds a bit more direct: if you leave...

It is a generic subject: you / one / we / people in general.

Russian often leaves the subject unstated when the meaning is general and not tied to one specific person. In this sentence, the speaker is talking about what should be done in the space, not about who exactly does it.

Оставить is perfective, while оставлять is imperfective.

Here the speaker is talking about the result of a completed action: leave at least 30 cm, and then the passage will be more convenient. Because the result matters, perfective is natural.

• оставить = to leave, as a completed act
• оставлять = to be leaving / to leave regularly / to leave as a process

If you used оставлять, it would sound more like a habitual or repeated situation.

The object is understood from context and does not need to be stated.

Here оставить means leave free / leave open / leave available, and the implied thing is something like:

• место
• пространство
• расстояние

So the idea is: leave at least thirty centimeters of space by the door.

Russian often omits an obvious object when it can be easily understood.

Хотя бы means at least or even just.

It sets a minimum acceptable amount:

• хотя бы тридцать сантиметров = at least thirty centimeters

It often adds the nuance of this much would already help.

After numbers, Russian changes the noun form depending on the numeral.

With 5 and higher, the noun is normally in the genitive plural:

• пять сантиметров
• десять сантиметров
• тридцать сантиметров

Compare:

• один сантиметр
• два / три / четыре сантиметра
• пять сантиметров

So тридцать сантиметров is exactly the form you expect after тридцать.

У takes the genitive case, and двери here is the genitive singular of дверь.

So:

• дверь = nominative
• у двери = by the door / near the door

In this sentence, у двери means by the door, near the door, or in the area of the door.

Проход is a noun meaning:

• passage
• way through
• walkway
• clearance

In this sentence it refers to the space people move through. So it is not the act of passing, but the physical passage area.

Удобнее is the comparative form of удобный and means more convenient / easier / more comfortable.

Russian very often uses the comparative directly after быть:

• будет удобнее = will be more convenient

This is natural and idiomatic.

A form like более удобный is possible in some contexts, but here удобнее is simpler and more natural.

Because the first part is a subordinate conditional clause:

• Если оставить хотя бы тридцать сантиметров у двери

and the second part is the main clause:

• проход будет удобнее

Russian normally separates an если clause from the main clause with a comma.

Yes. Russian word order is flexible, and changing it usually changes the focus or emphasis, not the basic meaning.

For example:

• Если у двери оставить хотя бы тридцать сантиметров, проход будет удобнее.

This is also natural.

The original version is neutral and straightforward. Moving у двери or хотя бы тридцать сантиметров can shift emphasis to the location or the amount.