Mi amiga dibujó un círculo casi perfecto, pero el ángulo del triángulo de al lado no era exacto.

Because amiga is the feminine form of amigo/amiga, meaning female friend.

• mi amigo = my male friend
• mi amiga = my female friend

Notice that mi does not change for masculine vs feminine singular nouns:

• mi amigo
• mi amiga

So the gender is shown by amigo/amiga, not by mi.

Dibujó is the third person singular preterite of dibujar:

• yo dibujé = I drew
• tú dibujaste = you drew
• él/ella dibujó = he/she drew

Here it means she drew.

The accent mark is important because it distinguishes dibujó from dibujo:

• dibujó = she/he drew
• dibujo = I draw / drawing

The accent also shows the stress falls on the last syllable: di-bu-jó.

Dibujó is the preterite, which usually presents the action as a completed event:

• Mi amiga dibujó un círculo = My friend drew a circle

This sounds like a finished action in a story.

Dibujaba is the imperfect, which would usually describe an ongoing, repeated, or background action:

• Mi amiga dibujaba un círculo = My friend was drawing a circle / used to draw a circle

So in your sentence, dibujó fits because the circle is presented as something she completed.

In Spanish, adjectives usually come after the noun, especially when they describe a literal quality:

• un círculo perfecto
• un ángulo exacto

The adverb casi goes with the adjective perfecto:

• casi perfecto = almost perfect

So the normal order is:

• un círculo casi perfecto

Putting the adjective before the noun is sometimes possible in Spanish, but it often sounds more literary, emotional, or stylistically marked. Here, the most natural wording is un círculo casi perfecto.

Because perfecto agrees with círculo, and círculo is a masculine singular noun.

Agreement in Spanish means adjectives must match the noun in gender and number:

• un círculo perfecto
• una figura perfecta
• unos círculos perfectos
• unas figuras perfectas

Even though amiga is feminine, that does not matter here. Perfecto describes círculo, not amiga.

Casi means almost.

In this sentence:

• un círculo casi perfecto = an almost perfect circle

It usually goes directly before the word or phrase it modifies. Here it modifies perfecto, so it comes before perfecto.

Compare:

• casi perfecto = almost perfect
• casi exacto = almost exact
• casi siempre = almost always

Pero means but, and it is the correct connector here.

The sentence contrasts two ideas:

• she drew an almost perfect circle,
• but the angle of the nearby triangle was not exact.

So:

• ..., pero ... = ..., but ...

Sino is used in a different structure, usually after a negative statement to correct or replace something:

• No era un círculo, sino un óvalo. = It wasn’t a circle, but rather an oval.

Your sentence is not correcting the first clause; it is simply contrasting it. So pero is right.

Because de + el contracts to del in Spanish.

So:

• de + el triángulo → del triángulo

This contraction is mandatory in normal Spanish.

Examples:

• la puerta del coche
• el color del libro
• el ángulo del triángulo

Important exception:

If el is part of a proper name, it does not contract:

• de El Escorial
  not del Escorial when the official name is El Escorial

But in your sentence, el triángulo is just a normal noun phrase, so del is required.

De al lado means next to it, beside it, or nearby, depending on context.

So:

• el triángulo de al lado = the triangle next to it / the nearby triangle

This is a very common Spanish expression:

• la casa de al lado = the house next door
• el coche de al lado = the car next to it
• la mesa de al lado = the table nearby / the next table

Here it identifies which triangle we mean: the one beside the circle or beside whatever is being referred to in the context.

Because de al lado functions like a modifier attached to the noun:

• el triángulo de al lado = the triangle next to it / the triangle nearby

If you say just al lado, you usually need a verb or a fuller structure:

• El triángulo está al lado. = The triangle is next to it / nearby.
• Está al lado del círculo. = It is next to the circle.

So:

• de al lado = attached to a noun
• al lado = often used after a verb

This is a classic imperfect vs preterite question.

• era = imperfect of ser
• fue = preterite of ser

In this sentence, no era exacto describes a quality of the angle: it wasn’t exact. The imperfect is very common for descriptions, states, and background information.

So the sentence is structured like this:

• dibujó = completed action
• no era exacto = descriptive state/quality

That contrast is very natural in Spanish storytelling.

If you said no fue exacto, it would sound more like you are treating being exact as a completed event or assessment, which is less natural here.

Spanish uses definite articles more often than English.

Here:

• el ángulo = the angle
• del triángulo = of the triangle

You are talking about a specific angle belonging to a specific triangle, so the definite article makes sense.

English sometimes drops articles where Spanish keeps them, but in this sentence both languages happen to use the:

• the angle of the triangle

Because exacto agrees with ángulo, and ángulo is masculine singular.

• el ángulo exacto
• la medida exacta

So in:

• el ángulo ... no era exacto

the adjective matches ángulo, not triángulo and not amiga.

No simply negates the verb:

• era exacto = it was exact
• no era exacto = it was not exact

In Spanish, no normally goes directly before the conjugated verb:

• no era
• no tenía
• no dibujó

That is one of the most basic and consistent negation patterns in Spanish.

Those written accents show the stressed syllable:

• círculo → CIR-cu-lo
• ángulo → AN-gu-lo
• triángulo → tri-AN-gu-lo

Without the accent marks, Spanish stress rules would make them sound different. The accents tell you that the stress falls earlier than the default rule would predict.

These words are all esdrújulas: the stress falls on the third-to-last syllable. In Spanish, all esdrújulas take a written accent.

They are pronounced differently because the stress changes:

• dibujo = di-BU-jo
• dibujó = di-bu-JÓ

That difference matters a lot:

• dibujo can mean I draw or drawing
• dibujó means he/she drew

So the accent mark is not optional; it changes both pronunciation and meaning.

Not exactly.

• perfecto = perfect
• exacto = exact, precise

They are related ideas, but not identical.

In your sentence:

• un círculo casi perfecto suggests the circle looked very good, very close to ideal
• el ángulo ... no era exacto focuses more on precision or measurement

So the sentence contrasts two different kinds of accuracy: one visual and one geometric/technical.

Yes, but it would slightly change the focus.

• un círculo casi perfecto = an almost perfect circle
  
  • almost modifies perfect
• casi un círculo perfecto = almost a perfect circle
  
  • almost modifies the whole noun phrase a bit more broadly

In many contexts the meaning is very similar, but un círculo casi perfecto is the more natural and standard phrasing here.

Yes. De al lado is very flexible and often means next door, next to it, or adjacent, depending on what noun it modifies.

Examples:

• la vecina de al lado = the neighbour next door
• la habitación de al lado = the room next door
• la mesa de al lado = the table next to ours / the nearby table

So in your sentence, el triángulo de al lado just means the triangle that is beside the relevant object in the context.

It has two main clauses joined by pero:

1. Mi amiga dibujó un círculo casi perfecto
2. pero el ángulo del triángulo de al lado no era exacto

So the pattern is:

• subject + verb + object
• pero
• subject + verb + complement

Breaking it down:

• Mi amiga = subject
• dibujó = verb
• un círculo casi perfecto = direct object

Then:

• el ángulo del triángulo de al lado = subject
• no era = verb phrase
• exacto = adjective/complement

This kind of sentence is very common in Spanish: one completed action followed by a contrasting description.