Me sorprendió lo fácil que fue coser la camisa con ayuda de mi abuela.

Me sorprendió means it surprised me. Spanish often expresses this idea with a verb that affects a person, rather than with an adjective.

• Me sorprendió... = It surprised me...
• Estaba sorprendido/a = I was surprised

Both can be correct in different contexts, but me sorprendió focuses on the event causing the reaction.

In this sentence, the thing that caused the surprise is: lo fácil que fue coser la camisa con ayuda de mi abuela

So the structure is basically:

• Me sorprendió X = X surprised me

Here, lo is a neuter article, and it is often used with an adjective to talk about the quality of being... or how...

So:

• lo fácil = how easy it was / the easy nature of it

This is a very common Spanish pattern:

• lo difícil = how difficult
• lo importante = what is important
• lo bueno = what is good

In your sentence, lo fácil que fue... means:

• how easy it was...

This lo does not mean it here.

The pattern lo + adjective + que + verb is very common in Spanish. It means how + adjective + subject + verb.

Examples:

• lo fácil que fue = how easy it was
• lo difícil que era = how difficult it was
• lo rápido que pasó el tiempo = how quickly time passed

So in your sentence:

• Me sorprendió lo fácil que fue coser la camisa...
• I was surprised by how easy it was to sew the shirt...

The que is just part of this fixed structure.

Both are possible in Spanish, but they give slightly different shades of meaning.

• fue (preterite) presents it as a completed event or overall result
• era (imperfect) presents it more as background, description, or an ongoing situation

Here, lo fácil que fue coser la camisa sounds like the speaker is looking back on a finished task and judging the whole experience. That makes fue very natural.

• fue = it turned out to be / it was (in that completed instance)
• era = it was (generally / as background / in progress)

Since sewing the shirt is seen as a completed action, fue fits well.

After expressions like fue fácil, Spanish often uses an infinitive to say to do something.

So:

• fue coser la camisa = it was to sew the shirt
• more natural English: it was to sew the shirt / it was sewing the shirt / it was to sew the shirt that was easy

In practice, Spanish often uses the infinitive where English might use to + verb:

• Es difícil entenderlo. = It’s difficult to understand it.
• Fue fácil hacerlo. = It was easy to do it.

So coser is there because it names the action: to sew.

Yes, both are grammatically possible, but the nuance changes.

• Me sorprendió lo fácil que fue coser la camisa
  This sounds like a completed experience: the sewing happened, and the speaker is reflecting on it.

• Me sorprendió lo fácil que era coser la camisa
  This sounds more descriptive, like the speaker is focusing on the process or general situation while it was happening.

In most everyday contexts about one finished task, fue sounds more natural.

Spanish uses definite articles more often than English.

So even when English might say:

• sew a shirt
• sew the shirt

Spanish often prefers:

• coser la camisa

Here, la camisa probably refers to the specific shirt involved in the situation. It sounds natural and complete in Spanish.

Leaving out the article (coser camisa) would sound wrong here.

Because the meaning is specifically with my grandmother’s help, not just with my grandmother.

Compare:

• con ayuda de mi abuela = with the help of my grandmother
• con mi abuela = with my grandmother

The second one only tells you she was present or involved somehow.
The first one clearly says she helped.

So con ayuda de is more precise.

Yes. Both are correct:

• con ayuda de mi abuela
• con la ayuda de mi abuela

The version without the article is very common and slightly more compact.
The version with la can sound a little more explicit or formal.

In everyday Spanish, con ayuda de is completely natural.

The subject is the whole clause:

lo fácil que fue coser la camisa con ayuda de mi abuela

That whole idea is what surprised the speaker.

So structurally, it is:

• [Me] sorprendió [lo fácil que fue coser la camisa con ayuda de mi abuela].

Literally:

• How easy it was to sew the shirt with my grandmother’s help surprised me.

Spanish often puts the indirect object pronoun first (me) and the real subject after the verb.

Both are possible.

• Me sorprendió lo fácil que fue...
  This is very natural and common. It puts the reaction first.

• Lo fácil que fue... me sorprendió
  Also correct, but it sounds more literary or more emphatic on the thing that caused surprise.

Spanish word order is more flexible than English. In this sentence, starting with me sorprendió sounds especially natural in conversation.

No. They are different verbs:

• coser = to sew
• cocer = to cook / boil

This is a very common confusion for learners.

In Spain, they are usually pronounced differently:

• coser → with an s sound
• cocer → with a th sound (like thin) in most of Spain

So in Spain Spanish, they are easier to tell apart by pronunciation than in many Latin American accents.

Not naturally here.

• fue coser la camisa works because the infinitive names the action
• fue cosiendo la camisa would not fit this structure

After lo fácil que fue, Spanish normally uses the infinitive for the activity being evaluated:

• Fue fácil hacerlo
• Fue difícil aprenderlo
• Fue interesante verlo

So coser is the natural form here.

Only the second type works.

• Fue fácil coser la camisa = It was easy to sew the shirt
• Me sorprendió lo fácil que fue coser la camisa = I was surprised by how easy it was to sew the shirt

But:

• la camisa de coser does not mean the shirt to sew here

In Spanish, de + infinitive does not work that way in this sentence.
So the original structure with coser la camisa is the correct one.

The sentence does not say explicitly who did the sewing, only that it happened with the help of my grandmother.

In normal interpretation, the speaker probably means:

• I sewed the shirt, and my grandmother helped me

But grammatically, the sentence focuses on the action itself, not on the exact subject doing it.

If you wanted to make it explicit, you could say:

• Me sorprendió lo fácil que me fue coser la camisa con ayuda de mi abuela.
• I was surprised by how easy it was for me to sew the shirt with my grandmother’s help.

That version makes the speaker’s involvement clearer.

Because this is one continuous sentence with no separate clause that needs a comma.

The structure is:

• Me sorprendió
  • what surprised me

And the rest of the sentence is one unit:

• lo fácil que fue coser la camisa con ayuda de mi abuela

In standard Spanish punctuation, no comma is needed here.