Meine These ist, dass regelmäßige Pausen beim Lernen helfen, und die Schlussfolgerung schreibe ich erst am Ende.

Because These is a feminine singular noun, and here it is the subject of the main clause, so it is in the nominative case.

The possessive mein- changes its ending depending on case, gender, and number:

• mein Satz = my sentence
• meine These = my thesis/claim
• meinen Satz = my sentence, as a masculine accusative object

So in this sentence, meine These is correct because it means my thesis/claim as the subject.

Here, These means something like claim, proposition, or main argument.

It does not usually mean a thesis paper in the sense of a university dissertation. For that, German more often uses words like:

• Bachelorarbeit
• Masterarbeit
• Dissertation

So Meine These ist, dass ... means something like My claim is that ... or My thesis is that ...

Dass introduces a subordinate clause meaning that.

In German, when a clause starts with a subordinating conjunction like dass, the finite verb usually goes to the end of that clause.

So:

• Main clause: Regelmäßige Pausen helfen beim Lernen.
• Subordinate clause: ..., dass regelmäßige Pausen beim Lernen helfen.

That is why helfen appears at the end.

Because there is no article before Pausen, the adjective has to carry the grammatical ending itself.

Here:

• Pausen is plural
• it is the subject of the dass-clause
• so it is in the nominative plural

With no article, the adjective takes the strong ending -e in nominative plural:

• regelmäßige Pausen
• lange Texte
• frische Brötchen

So regelmäßige Pausen is the correct form.

Beim Lernen means while learning, when learning, or during studying.

It is made from:

• bei dem → beim
• Lernen = learning, used here as a noun

So literally it is something like during the learning.

This is a very common German structure:

• beim Arbeiten = while working
• beim Kochen = while cooking
• beim Lesen = while reading

Because here Lernen is being used as a noun, not as a normal verb.

In German, nouns are capitalized. A verb can become a noun when it is used in this way, often after an article or a preposition:

• lernen = to learn
• das Lernen = learning
• beim Lernen = while learning

So the capital L shows that it is a nominalized verb.

Because the subject of that clause is regelmäßige Pausen, which is plural.

German verbs have to agree with their subject:

• Die Pause hilft. = The break helps.
• Die Pausen helfen. = The breaks help.

So with Pausen, the correct verb form is helfen.

Because German main clauses follow the verb-second rule.

That means the finite verb must come in the second position, but the first position can be taken by different sentence elements.

Here, die Schlussfolgerung is placed first for emphasis or topic:

• Die Schlussfolgerung | schreibe | ich | erst am Ende

This is also correct:

• Ich schreibe die Schlussfolgerung erst am Ende.

The difference is mostly emphasis:

• Ich schreibe ... = more neutral
• Die Schlussfolgerung schreibe ich ... = puts extra focus on the conclusion

No. Here erst does not mean first.

In this sentence, erst means something like:

• only
• not until

So erst am Ende means only at the end or not until the end.

Compare:

• zuerst = first
• erst am Ende = only at the end
• Ich komme erst morgen. = I’m not coming until tomorrow.

That is a very common use of erst in German.

The comma is there because the dass-clause must be closed off with a comma.

Structure:

• Meine These ist, dass regelmäßige Pausen beim Lernen helfen, und ...

The important point is that the comma before und is really the closing comma for the subordinate clause dass regelmäßige Pausen beim Lernen helfen.

So it is not mainly there because of und. It is there because the dass-clause has to be separated from the rest of the sentence.

Ist is the verb sein in the 3rd person singular.

The main clause is:

• Meine These ist ...

Then the dass-clause gives the content of that thesis:

• ..., dass regelmäßige Pausen beim Lernen helfen

So the whole structure means:

• My thesis is that ...

This is a very common pattern in German:

• Mein Eindruck ist, dass ...
• Das Problem ist, dass ...
• Die Wahrheit ist, dass ...