Nous avons commencé par le bloc, n’est-ce pas ?

It is the passé composé, a very common French past tense used for completed actions.

• nous avons = the present tense of avoir
• commencé = the past participle of commencer

So nous avons commencé literally means we have started / we started.

In natural English, it often translates simply as we started.

In the passé composé, most verbs use avoir as the auxiliary verb. Commencer is one of them.

So:

• j’ai commencé = I started
• tu as commencé = you started
• nous avons commencé = we started

A smaller group of verbs uses être instead, but commencer is not one of those.

Because with avoir, the past participle usually does not agree with the subject.

So even though nous means we, you still normally say:

• Nous avons commencé

not

• Nous avons commencés

Agreement with the past participle usually happens with avoir only if there is a direct object placed before the verb.

Commencer par means to begin with or to start with.

So:

• Nous avons commencé par le bloc = We started with the block

This is a fixed and very common pattern:

• commencer par quelque chose = to start with something
• commencer par faire quelque chose = to start by doing something

Examples:

• Je vais commencer par l’introduction. = I’m going to start with the introduction.
• On a commencé par discuter du problème. = We started by discussing the problem.

Because commencer par is the normal French expression for to start with in the sense of doing this first.

• par suggests the first step or starting point
• avec usually means with in a more general sense, but it is not the usual choice after commencer for this meaning

So French prefers:

• commencer par le bloc

rather than:

• commencer avec le bloc (generally unnatural here)

Grammatically, bloc is a masculine singular noun, so it takes le.

Its exact meaning depends on context. It could mean:

• a block
• a section
• a unit
• a block in a game, lesson, exercise, building context, etc.

Since you said the meaning is already shown to the learner, the main grammar point is just:

• le = masculine singular definite article
• bloc = masculine noun

N’est-ce pas ? is a tag question, similar to English:

• isn’t it?
• right?
• don’t we?
• didn’t we? depending on context

In this sentence, it is being used to ask for confirmation:

• Nous avons commencé par le bloc, n’est-ce pas ?
• We started with the block, right?

French often uses n’est-ce pas ? as a general confirmation tag, even when English would choose a more specific tag like didn’t we?

French can form more specific tag-like endings in speech, but n’est-ce pas ? is a very common all-purpose way to ask for confirmation.

English tag questions change depending on the subject and tense:

• We started..., didn’t we?
• She is..., isn’t she?
• They can..., can’t they?

French often avoids matching all those details and simply uses:

• n’est-ce pas ?

So it is less literal and more flexible than English tag questions.

The apostrophe shows elision.

• ne est ce would be awkward in French
• so ne becomes n’ before a vowel sound

That gives:

• n’est-ce pas ?

This is very common in French:

• je aime → j’aime
• ne a → n’a
• si il → s’il

Here, ce is part of the fixed expression n’est-ce pas ?

Literally, it looks like:

• is it not so?
• is that not the case?

But you should really learn n’est-ce pas ? as a whole expression meaning:

• right?
• isn’t that so?

So while ce often means this or it, in this phrase it is best understood as part of a set expression rather than translated word by word.

A careful pronunciation would be roughly:

• Nous avons commencé par le bloc, n’est-ce pas ?
• noo-zah-von co-mon-say par luh blok, ness-pah?

A few helpful points:

• nous avons usually has liaison: noo-zah-von
• commencé ends with an ay sound
• bloc has a clear final k sound
• n’est-ce pas is often pronounced smoothly, almost like ness-pah

The final rise in intonation helps show that it is a confirmation question.

Yes. In everyday spoken French, on often replaces nous.

So both are correct:

• Nous avons commencé par le bloc.
• On a commencé par le bloc.

The difference is mostly style:

• nous is a bit more formal or careful
• on is more common in everyday conversation

Both can mean we started with the block.

Yes. The main statement has standard word order:

• Nous = subject
• avons commencé = verb
• par le bloc = prepositional phrase

Then n’est-ce pas ? is added at the end as a tag.

So the structure is:

• statement + confirmation tag

This is very normal in French:

• Il est prêt, n’est-ce pas ?
• Vous avez compris, n’est-ce pas ?
• On commence demain, n’est-ce pas ?