No pertenezco a este grupo.

Because pertenecer is a verb that normally takes the preposition a in Spanish.

• pertenecer a algo/alguien = to belong to something/someone

So:

• Pertenezco a este grupo = I belong to this group
• No pertenezco a este grupo = I do not belong to this group

The a is required here; leaving it out would sound wrong.

Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

• pertenezco = I belong

The ending -ezco tells you it is yo, so yo is optional.

You could say:

• No pertenezco a este grupo
• Yo no pertenezco a este grupo

Both are correct. Adding yo gives extra emphasis, as in I don’t belong to this group.

Pertenezco is the first person singular present tense form of pertenecer.

Conjugation in the present:

• yo pertenezco
• tú perteneces
• él/ella/usted pertenece
• nosotros/nosotras pertenecemos
• vosotros/vosotras pertenecéis
• ellos/ellas/ustedes pertenecen

So in the sentence, pertenezco means I belong.

This is a spelling change found in some verbs ending in -ecer, -acer, -ocer, and -ucir.

In the yo form, they often change to -zco:

• pertenecer → pertenezco
• conocer → conozco
• parecer → parezco

This is not random: it is part of the standard conjugation pattern for these verbs.

Important: the change happens only in certain forms, not in all of them:

• yo pertenezco
• but tú perteneces, not pertenezces

In Spanish, basic negation is usually formed by putting no directly before the conjugated verb.

• Pertenezco = I belong
• No pertenezco = I do not belong

This is the normal word order in simple negative sentences.

Because grupo is a masculine singular noun, so the demonstrative must match it.

• este = masculine singular
• esta = feminine singular

So:

• este grupo = this group
• esta clase = this class

The ending -o in grupo is a strong clue that it is masculine, though not every noun follows that pattern.

Este means this.

It points to something close to the speaker, either physically or mentally.

So:

• este grupo = this group
• ese grupo = that group (near the listener or less close to the speaker)
• aquel grupo = that group over there / that group farther away

In everyday use, este is the natural choice for this.

In standard Spain Spanish, pertenezco is pronounced approximately:

pehr-teh-NEHTH-koh

More precisely, the z in Spain is usually pronounced like the th in think, so:

• pertenezco → per-te-NEZ-co in much of Latin America
• pertenezco → per-te-NETH-co in most of Spain

Also:

• the stress falls on -nez-
• the r is a light tap
• the c before o sounds like k

Pertenecer is a very natural and correct verb for expressing membership or belonging.

• No pertenezco a este grupo = I do not belong to this group

Depending on context, Spanish could also use other expressions, but they are not always exact matches:

• No soy de este grupo = I’m not from this group / I’m not part of this group
  This can work in some situations, but it is less direct and not always identical.
• No formo parte de este grupo = I am not part of this group
  This is also correct and a bit more formal or explicit.

So pertenecer a is a very good core structure to learn.

No. Grupo can refer to many kinds of groups, such as:

• a social group
• a work team
• a political group
• a chat group
• a music group/band
• a category or classification

So No pertenezco a este grupo can be used in both literal and figurative situations.

No, not with pertenecer.

English learners sometimes want to use en because English says in this group, but Spanish uses a with this verb:

• correct: pertenecer a un grupo
• incorrect: pertenecer en un grupo

This is a good example of a verb-preposition combination that has to be learned as a unit: pertenecer a.

Yes, depending on context, it can sound slightly broader than formal membership.

It can mean:

• I am not a member of this group
• I do not belong to this group
• I am not part of this group

Sometimes it can even suggest emotional or social non-belonging, not just official membership. The exact nuance depends on the situation.

It is neutral and works well in both formal and informal contexts.

It is not slangy, and it is not especially stiff either. You could use it in:

• conversation
• writing
• workplace situations
• school or academic contexts

If you wanted something slightly more formal or explicit, you could say:

• No formo parte de este grupo

But No pertenezco a este grupo is already perfectly normal and natural.