La llanta gira alrededor del eje.

Because de + el contracts to del in Spanish.

So:

• de el eje → del eje

This contraction is mandatory in normal Spanish whenever de is followed by the masculine singular article el.

A similar contraction happens with:

• a + el → al

But note that this only applies when el is the article. If Él means he, there is no contraction.

Because the expression is alrededor de = around.

So the structure is:

• alrededor de + noun

Examples:

• alrededor de la casa
• alrededor del eje
• alrededor de la mesa

Since eje is masculine singular and takes el, de + el becomes del.

Because the subject is singular: La llanta.

• La llanta = singular
• therefore the verb must also be singular: gira

Compare:

• La llanta gira = The wheel/rim turns
• Las llantas giran = The wheels/rims turn

Spanish usually does not use subject pronouns when the subject is already clear.

In English, you would need it turns.
In Spanish, the subject can simply be the noun itself:

• La llanta gira

You could think of this as:

• The wheel turns

rather than needing a separate it.

Spanish does have pronouns like él and ella, but they are normally omitted unless needed for emphasis or contrast.

Spanish often uses the definite article more often than English does.

Here, La llanta refers to the wheel/rim as the subject of the sentence. That is completely normal Spanish.

Also, Spanish commonly uses articles with parts of machines, objects, and the body, where English might sometimes choose a possessive or leave things more implicit.

So La llanta gira alrededor del eje sounds natural as a general statement about that object.

This is a very good question, because vocabulary can vary by region.

In Spain, llanta usually means rim rather than the whole wheel or the tire. Depending on context:

• llanta = rim
• neumático = tire
• rueda = wheel

So in Spain, if you mean the whole wheel, La rueda gira alrededor del eje may sound more natural.
If you specifically mean the metal rim, then llanta is fine.

In some parts of Latin America, llanta can refer to the tire or wheel more broadly, so usage differs.

Because girar can be used as a normal intransitive verb here.

• La llanta gira = The wheel/rim rotates

Using se would change the feel or meaning and is not the normal choice for this basic physical description.

Very roughly:

• girar = to turn, rotate
• girar algo = to turn something
• girarse can mean to turn oneself around, turn around, or shift position depending on context

For an object rotating around an axis, gira is the straightforward option.

The simple present in Spanish is often used for:

• general truths
• descriptions
• habitual actions
• neutral statements about what something does

So La llanta gira alrededor del eje works well for a general mechanical description.

If you said La llanta está girando alrededor del eje, that would emphasize that the turning is happening right now.

So:

• gira = turns / rotates / does turn
• está girando = is turning / is rotating right now

Usually approximately as YAN-ta in most modern Spanish pronunciation.

A few points:

• ll is commonly pronounced like the y in yes in much of Spain
• a is a short, clear vowel
• the stress is on the first syllable: LLAN-ta

In some regions or among some speakers, ll may sound slightly different, but for most learners, YAN-ta is a good practical guide.

Eje is pronounced roughly like EH-heh, but with the Spanish j sound.

Important details:

• e = like e in bed, but cleaner and tenser
• j = a throaty sound, like the ch in Scottish loch or German Bach
• stress is on the first syllable: E-je

So it is not like the English j in jam.

Yes, but the original order is the most neutral:

• La llanta gira alrededor del eje.

Spanish word order is flexible, but changes can sound marked, literary, or emphatic.

For example:

• Alrededor del eje gira la llanta.

This is grammatically possible, but it sounds more emphatic or stylistically marked. For a normal statement, the original sentence is best.

Yes. En torno a is another way to say around in this context.

So you could say:

• La llanta gira alrededor del eje.
• La llanta gira en torno al eje.

Both are correct.
Alrededor de is often the more direct and common choice for learners, while en torno a can sound slightly more formal or technical depending on context.