Sa akin ang bolpen na nasa tabi ng kuwaderno.

Because Filipino often expresses possession with a phrase like:

• Sa akin ang ... = ... is mine / belongs to me
• literally, it is something like To me is the pen...

So Sa akin ang bolpen... is a very natural way to say that the pen belongs to the speaker.

You may also see:

• Akin ang bolpen...

This means basically the same thing.
Akin is a shorter possessive form, while sa akin is also very common and often feels a bit more conversational or natural in many contexts.

Ang marks the focused or topic noun phrase of the sentence.

Here, the phrase marked by ang is:

• ang bolpen na nasa tabi ng kuwaderno

So the sentence is structurally like:

• Sa akin = predicate/comment
• ang bolpen na nasa tabi ng kuwaderno = topic/subject-like part

A rough pattern is:

• Sa akin ang X = X is mine

Even though English uses the pen as the grammatical subject, Filipino uses ang to mark the noun phrase being talked about.

Here, na is a linker. It connects bolpen to the description that follows it.

So:

• bolpen na nasa tabi ng kuwaderno = the pen that is beside the notebook

The linker is very common in Filipino. It joins a noun to an adjective, phrase, or clause that describes it.

Compare:

• magandang bahay = beautiful house
• bahay na maganda = house that is beautiful

In your sentence, na links:

• bolpen
  • nasa tabi ng kuwaderno

Nasa usually means is in / is at / is on / is located at.

It comes historically from na sa, but learners usually just treat nasa as a common unit.

In this sentence:

• nasa tabi ng kuwaderno = is beside the notebook

So:

• sa by itself is a preposition like in, at, to, on
• nasa often works like is in/at/on

Examples:

• sa mesa = on the table / at the table
• nasa mesa = is on the table

That is why nasa fits well inside a descriptive clause:

• bolpen na nasa tabi ng kuwaderno = pen that is beside the notebook

Because tabi commonly takes ng when it is followed by the thing something is next to.

So the pattern is:

• sa tabi ng X = beside X / next to X

Examples:

• sa tabi ng bahay = beside the house
• sa tabi ng pinto = beside the door
• sa tabi ng kuwaderno = beside the notebook

Here:

• sa marks the location phrase
• tabi means side
• ng kuwaderno tells what it is beside

So nasa tabi ng kuwaderno literally feels like is at the side of the notebook.

Filipino does not always follow the same subject-verb-object style that English does.

This sentence is not built like The pen ... is mine in the same order. Instead, it follows a very common Filipino pattern where the predicate comes first:

• Sa akin = mine / belongs to me
• ang bolpen na nasa tabi ng kuwaderno = the pen beside the notebook

So the structure is closer to:

• Mine is the pen beside the notebook

That sounds unusual in English, but it is normal in Filipino.

Filipino often puts:

1. the comment/predicate first
2. the ang-phrase after it

Yes. That is also correct.

This version uses ay, which often appears when the ang-phrase is moved to the front:

• Ang bolpen na nasa tabi ng kuwaderno ay sa akin.

This is more like:

• The pen beside the notebook is mine.

So you can compare:

• Sa akin ang bolpen na nasa tabi ng kuwaderno.
  Very natural, common predicate-first order.

• Ang bolpen na nasa tabi ng kuwaderno ay sa akin.
  Also correct; more formal or more explicitly fronting the noun phrase.

Kuwaderno means notebook.

It is a standard Filipino spelling based on a borrowed word from Spanish cuaderno.

Learners often notice that the spelling looks different from English. That is normal in Filipino, because borrowed words are often written according to Filipino spelling conventions:

• cuaderno → kuwaderno
• ballpen → bolpen

So kuwaderno is a normal Filipino word, not a typo.

Yes. Bolpen is a very common borrowed word meaning ballpen or pen.

In everyday speech, many Filipinos use:

• bolpen = pen
• sometimes ballpen in more English-influenced speech

So in this sentence, bolpen is completely natural.

A more general native-style word like panulat exists, but it is broader and can mean a writing instrument in general, not specifically a ballpoint pen.

Because the linker na attaches the descriptive part directly to bolpen.

So this chunk:

• bolpen na nasa tabi ng kuwaderno

works as one noun phrase:

• the pen that is beside the notebook

The relative clause nasa tabi ng kuwaderno is modifying bolpen.

If you stop after ang, the whole thing that follows up to the end is the item being identified as belonging to the speaker.

Yes. Sa akin is a very flexible phrase, and its meaning depends on context.

It can mean things like:

• to me
• for me
• at my place
• with me
• mine (in possession/ownership sentences like this one)

Examples:

• Ibigay mo sa akin. = Give it to me.
• Sa akin ka na lang umupo. = Sit next to me / by me.
• Sa akin ang librong ito. = This book is mine.

So in your sentence, the structure tells you that sa akin is expressing possession.

You can replace sa akin with other pronoun forms.

Common ones:

• sa iyo / sa'yo = yours
• sa kanya = his/hers
• sa amin = ours, not including the listener
• sa atin = ours, including the listener
• sa inyo = yours plural / yours formal
• sa kanila = theirs

Examples:

• Sa iyo ang bolpen. = The pen is yours.
• Sa kanya ang bolpen. = The pen is his/hers.
• Sa amin ang bolpen. = The pen is ours.

You can also use the short possessive forms:

• akin = mine
• iyo = yours
• kanya = his/hers
• amin = ours
• atin = ours incl.
• inyo = yours plural/formal
• kanila = theirs

So:

• Akin ang bolpen.
• Iyo ang bolpen.
• Kanya ang bolpen.

are also correct.

Usually, Sa akin ang bolpen... strongly suggests possession in the sense of it is mine / it belongs to me.

However, in real conversation, context matters. Sometimes it can mean something closer to:

• I have the pen
• the pen is with me
• that pen is mine

But in a neutral sentence like this, most learners should understand it as ownership or clear possession.

If you specifically wanted to say I have the pen, Filipino often uses a different structure, such as:

• Nasa akin ang bolpen.

That can mean The pen is with me rather than The pen is mine.

So compare:

• Sa akin ang bolpen. = The pen is mine.
• Nasa akin ang bolpen. = The pen is with me.

That difference is very useful.

A helpful way to break it down is:

• Sa akin = mine / belongs to me
• ang bolpen = the pen
• na nasa tabi ng kuwaderno = that is beside the notebook

So the whole sentence is:

• Sa akin
  ang bolpen
  na nasa tabi ng kuwaderno

Seeing it in chunks makes the grammar much easier to follow.