El alfiler está junto al descosedor.

Because this sentence is talking about location: where the pin is.

In Spanish, estar is normally used for location:

• El alfiler está junto al descosedor. = The pin is next to the seam ripper.

Ser is used for identity, description, origin, etc., not for physical location in cases like this.

So:

• está = is in the sense of is located
• es would sound wrong here

Junto a means next to, beside, or close to.

In this sentence:

• junto al descosedor = next to the seam ripper

The al part is made from:

• a + el = al

So:

• junto a el descosedor is not correct
• it must become junto al descosedor

In Spanish, when the preposition a is followed by the masculine singular article el, they normally contract:

• a + el = al
• de + el = del

So:

• junto al descosedor is correct
• junto a el descosedor is not standard Spanish

This contraction happens automatically unless el is part of a proper name.

Alfiler means pin.

It is a masculine noun, so it takes:

• el alfiler

Its gender is something you mostly just have to learn with the noun. The ending -er does not always tell you the gender with certainty, but alfiler is masculine.

So:

• el alfiler
• un alfiler

A descosedor is a sewing tool used to unpick stitches — in English, usually a seam ripper.

The word comes from the verb descoser, meaning to unstitch or to unsew.

In Spain, descosedor is a normal word for this tool. Depending on region or context, you may also hear other sewing-related terms, but descosedor is perfectly natural.

Spanish uses the definite article el / la / los / las very often, and in this sentence both objects are being referred to as specific things:

• El alfiler = the pin
• el descosedor = the seam ripper

That is completely normal Spanish.

Spanish often uses articles more consistently than English, so learners should not expect article use to match English exactly in every sentence.

Yes. That would be very natural too.

• El alfiler está junto al descosedor.
• El alfiler está al lado del descosedor.

Both mean The pin is next to the seam ripper.

A small difference:

• junto a can feel a bit more like right next to / close beside
• al lado de is also very common and often the most straightforward way to say next to / beside

In many everyday situations, they are interchangeable.

In standard Spain Spanish, it is roughly:

• des-co-se-DOR

More carefully:

• des as in dess (with a Spanish d and s)
• co = koh
• se = seh
• dor = DOR, with the stress on the last syllable

Because the word ends in -r, the stress naturally falls on the last syllable:

• des-co-se-DOR

And está has a written accent to show the stress:

• es-TÁ

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

Standard order:

• El alfiler está junto al descosedor.

You could also change it for emphasis, for example:

• Junto al descosedor está el alfiler.

That sounds more marked, like you are pointing out where the pin is. For a basic descriptive sentence, the original word order is best.

The sentence changes to match number and gender.

For example:

• Los alfileres están junto al descosedor. = The pins are next to the seam ripper.
• La aguja está junto al descosedor. = The needle is next to the seam ripper.
• Las agujas están junto al descosedor. = The needles are next to the seam ripper.

Notice:

• el / la / los / las change with the noun
• está / están change with singular or plural
• junto al descosedor stays the same here because descosedor is still masculine singular

Normally, when junto means next to, it is used as junto a:

• junto al descosedor
• junto a la caja
• junto a mí

In everyday learning, it is best to treat junto a as a set expression meaning next to / beside.

You may sometimes see junto con, but that means together with, which is different:

• Vino junto con su hermana. = He/She came together with his/her sister.

So in this sentence, junto a is the right pattern.