Mi amigo es bienvenido aquí.

In modern Spanish, possessive adjectives like mi, tu, su, nuestro normally replace the article. So mi amigo means my friend.

Using el mi amigo is not standard in normal modern Spanish.

Usually, yes. Amigo is masculine singular.

• If the friend is female, say Mi amiga es bienvenida aquí.
• If it is a group, use the plural:
  • Mis amigos son bienvenidos aquí
  • Mis amigas son bienvenidas aquí

So both the noun and bienvenido change to match gender and number.

Spanish normally uses ser in this expression: ser bienvenido = to be welcome.

Here, the sentence is talking about your friend being accepted or invited in this place, not about his physical location or a temporary condition. That is why es is used.

So:

• Mi amigo es bienvenido aquí = My friend is welcome here

Using está here would not be the normal choice.

Here, bienvenido functions like an adjective meaning welcome.

It describes mi amigo, so it must agree with the noun:

• bienvenido — masculine singular
• bienvenida — feminine singular
• bienvenidos — masculine plural / mixed plural
• bienvenidas — feminine plural

That is why the form is bienvenido with amigo.

Yes, it is grammatical and natural, but it can sound a little formal or deliberate.

It is the kind of sentence you might use to reassure someone or state something clearly:

• Tu amigo es bienvenido aquí
• Siempre será bienvenido aquí

In very casual conversation, people might also say something more idiomatic depending on the situation, such as:

• Tu amigo puede venir aquí cuando quiera
• Tu amigo siempre puede pasar por aquí

But the original sentence is absolutely correct.

Aquí means here, and putting it at the end is a very normal word order in Spanish.

The sentence flows naturally as:

• Mi amigo = subject
• es bienvenido = is welcome
• aquí = here

You can move aquí for emphasis in some contexts, but the version with aquí at the end is the most neutral and common.

Yes, in much of Latin America, acá and aquí can often both mean here.

So you could also say:

• Mi amigo es bienvenido acá

In many situations, the difference is small. Regional preference matters a lot. In general:

• aquí is very standard
• acá often sounds a bit more conversational in many Latin American varieties

Because mi amigo is the subject of the sentence, not the direct object.

The personal a is used before specific human direct objects, for example:

• Veo a mi amigo = I see my friend

But in Mi amigo es bienvenido aquí, mi amigo is the person being described by the verb es, so no personal a is used.

Yes. Spanish often uses Bienvenido, Bienvenida, Bienvenidos, or Bienvenidas by themselves as a greeting:

• ¡Bienvenido! = Welcome!
• ¡Bienvenidos! = Welcome! (to more than one person)

But in Mi amigo es bienvenido aquí, it is not a greeting. It is a full statement saying that your friend is welcome here.

You change the noun, the verb, and bienvenido so they agree:

• Mis amigos son bienvenidos aquí = My friends are welcome here
• Mis amigas son bienvenidas aquí = My female friends are welcome here

So Spanish agreement is important across the whole sentence.

Because the standard spelling of the word is aquí, with an accent on the í.

You should write the accent mark in normal Spanish writing. It is part of the correct spelling of the word.