Nel quaderno lascio sempre un margine a sinistra.

Nel is the contraction of in + il.

• in il quaderno → nel quaderno

Italian very often combines a preposition with the definite article:

• in + il = nel
• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

So nel quaderno means in the notebook.

Quaderno usually means a notebook, especially a school-style notebook or exercise book.

It is different from some other English uses of notebook, because in Italian:

• quaderno = notebook/exercise book
• taccuino = notepad / pocket notebook
• diario = diary / planner / journal

So in this sentence, quaderno suggests the kind of notebook you write in on pages.

Lascio is the first person singular of lasciare in the present indicative.

So:

• io lascio = I leave
• tu lasci = you leave
• lui/lei lascia = he/she leaves

In this sentence, lascio means I leave in the sense of I leave some space.

Italian often omits subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• lascio already tells you the subject is io = I

So:

• Io lascio sempre un margine a sinistra
• Lascio sempre un margine a sinistra

Both are correct, but the second is more natural unless you want emphasis.

You would add io if you want contrast or emphasis, for example:

• Io lascio sempre un margine, lui no.
• I always leave a margin, he doesn’t.

Sempre means always, and its position is fairly flexible, but after the verb is very common and natural here:

• Lascio sempre un margine a sinistra.

This sounds like: I always leave a left margin / I always leave a margin on the left.

Other placements are possible, but they may change the emphasis slightly:

• Sempre lascio un margine a sinistra = more marked, less neutral
• Lascio un margine sempre a sinistra = possible, but less natural in this context

So the given word order is the most normal one.

Un margine uses the indefinite article, so it means a margin rather than the margin.

That makes sense because the sentence is talking about the general habit of leaving some margin space, not a specific previously mentioned margin.

• un margine = a margin
• il margine = the margin

Also, margine is a masculine singular noun, so the article is un.

Margine is masculine:

• il margine
• un margine
• i margini

This can be tricky because many nouns ending in -e can be either masculine or feminine, so you usually have to learn the gender with the noun.

Here a sinistra means on the left or to the left.

In this sentence, it describes where the margin is:

• un margine a sinistra = a left-hand margin / a margin on the left

It is a very common expression, just like:

• a destra = on the right
• a sinistra = on the left

Because a sinistra is the normal fixed expression for on the left / to the left.

• a sinistra = idiomatic, general location or direction
• alla sinistra = much less common in this kind of sentence; it usually sounds more specific or depends on another noun/pronoun

For example:

• Gira a sinistra. = Turn left.
• C’è un margine a sinistra. = There is a margin on the left.

So in your sentence, a sinistra is exactly what Italian normally uses.

It is most naturally understood as describing margine.

So the idea is:

• un margine a sinistra = a margin on the left

Not really:

• I leave ... on the left in some separate sense

In other words, the sentence means that the margin is located on the left side of the page.

Yes, but in a very common extended sense.

In Italian, lasciare can mean:

• to leave
• to let
• to leave behind
• to leave something empty/free

Here it means something like:

• to leave space
• to keep a margin
• to leave blank

So lascio sempre un margine a sinistra means that when writing, the speaker always keeps some blank space on the left side.

Yes, but the original order is very natural and neutral.

Original:

• Nel quaderno lascio sempre un margine a sinistra.

Possible alternatives:

• Lascio sempre un margine a sinistra nel quaderno.
• Sempre nel quaderno lascio un margine a sinistra.

These are grammatically possible, but they shift emphasis a bit.

Starting with Nel quaderno puts the setting first: In the notebook, ... That can sound very natural if you are talking about where this habit happens.

Putting Nel quaderno first gives the sentence a natural topic-first structure.

It is a bit like saying:

• As for the notebook, I always leave a margin on the left.
• In the notebook, I always leave a margin on the left.

Italian often moves location expressions to the beginning when they set the scene or context.

So this word order is not unusual at all.

Usually no, not if you mean writing inside a notebook.

• nel quaderno = in the notebook, inside it, on its pages
• sul quaderno = on the notebook, on top of it, on its cover/surface

Since you are talking about leaving a margin on the page, nel quaderno is the correct choice.