Allontano la sedia dalla porta.

Here allontano is the 1st person singular, present tense form of allontanare, meaning to move something away, to take something away, or to distance something.

So:

• allontano = I move away / I take away / I move farther

In this sentence, the subject I is not written because Italian normally leaves subject pronouns out when the verb already makes the person clear.

Because Italian is a pro-drop language: the verb ending often already tells you who is doing the action.

• allontano = I move away
• allontani = you move away
• allontana = he/she moves away

So Io allontano la sedia dalla porta is possible, but io is usually unnecessary unless you want emphasis, contrast, or clarity.

For example:

• Io allontano la sedia, non tu. = I move the chair away, not you.

In Italian, articles are used more often than in English. With a normal concrete noun like chair, Italian usually wants the definite or indefinite article.

So:

• la sedia = the chair
• una sedia = a chair

Leaving the article out would usually sound unnatural here.

Dalla is the combination of:

• da = from / away from
• la = the

So:

• da + la = dalla

That is why dalla porta means from the door or away from the door.

Similar combinations are:

• dal = da + il
• dallo = da + lo
• dai = da + i
• dagli = da + gli
• dalle = da + le

Because the sentence expresses movement away from a place/object.

• da often means from, away from, or at/by depending on context.
• Here it marks separation: the chair is being moved away from the door.

So dalla porta is the natural choice.

Using di here would not express the same idea.

Here it is transitive, because it takes a direct object:

• la sedia = the thing being moved

So:

• Allontano la sedia = I move the chair away

There is also the reflexive form:

• allontanarsi = to move away / to go away

Compare:

• Allontano la sedia dalla porta. = I move the chair away from the door.
• Mi allontano dalla porta. = I move away from the door.

That is a very common distinction for learners to notice.

Italian present tense often covers both:

• I move the chair away from the door
• I am moving the chair away from the door

The exact English translation depends on context.

Italian does have ways to stress an ongoing action, such as:

• Sto allontanando la sedia dalla porta. = I am moving the chair away from the door.

But the simple present allontano is perfectly normal and often enough on its own.

Yes, in many contexts you could, but the nuance is a little different.

• spostare = to move / shift
• allontanare = to move away / make more distant

So:

• Sposto la sedia dalla porta focuses more on moving it.
• Allontano la sedia dalla porta focuses more on moving it farther away from the door.

If the idea of increasing the distance from the door matters, allontanare is especially appropriate.

Yes. Italian word order is flexible, though the most neutral order here is:

• Allontano la sedia dalla porta.

You could also hear:

• La sedia la allontano dalla porta.
  This gives extra emphasis to the chair.

• Dalla porta allontano la sedia.
  This is less neutral and sounds more marked or literary/context-driven.

For everyday use, the original order is the most natural.

Because la sedia is the direct object of the verb allontano.

The structure is:

• allontano = verb
• la sedia = direct object
• dalla porta = phrase showing what it is moved away from

In English, this works the same way:

• I move the chair away from the door

No preposition is needed before the chair.

Yes. Allontanare can be used both literally and figuratively.

Literal:

• Allontano la sedia dalla porta. = I move the chair away from the door.

Figurative:

• Allontano i dubbi. = I dispel doubts.
• Allontano una persona dalla mia vita. = I distance a person from my life.

In your sentence, because of la sedia and dalla porta, the meaning is clearly physical.

Because in Italian porta normally means door.

The English word port is usually:

• porto in Italian

So:

• la porta = the door
• il porto = the port / harbor

The article also helps:

• porta is feminine: la porta
• porto is masculine: il porto

A useful breakdown is:

• Allontano = verb, I move away
• la sedia = direct object, the chair
• dalla porta = prepositional phrase, away from the door

So the pattern is:

Verb + object + source/origin point

In a more abstract grammar description:

[verb] + [thing being moved] + [da + place/object it is moved away from]

That pattern is very common in Italian with verbs expressing separation or movement away.