Meine Mutter kauft immer dasselbe Waschmittel, weil es gut riecht.

Because meine Mutter is the subject of the sentence, so it is in the nominative case.

• meine Mutter = my mother
• She is the person doing the action: kauft

Meiner Mutter would be a different case, usually dative or genitive, depending on the sentence.

In German, all nouns are capitalized. So Mutter, Waschmittel, and any other noun begin with a capital letter.

This is one of the most noticeable spelling differences between German and English.

Dasselbe means the very same one.

So dasselbe Waschmittel means your mother always buys the exact same detergent, not just a similar one.

A useful comparison:

• dasselbe = the same exact one
• das gleiche = one that is the same type or kind

In everyday speech, Germans often use das gleiche where they also mean dasselbe, but the traditional distinction is still useful to know.

Because Waschmittel is a neuter singular noun: das Waschmittel.

The word dasselbe changes its form to match the noun’s gender, number, and case. Here the noun is:

• neuter
• singular
• accusative (because it is the direct object of kauft)

For neuter singular nominative and accusative, the form is dasselbe.

The noun is das Waschmittel, so it is neuter.

Because of that, the sentence refers back to it with es, which is the neuter pronoun meaning it.

So:

• das Waschmittel
• es

That is why the second clause says weil es gut riecht.

Immer is an adverb meaning always, and in a neutral German sentence it often appears in the middle of the sentence, after the conjugated verb and before the object.

So this word order is very natural:

• Meine Mutter kauft immer dasselbe Waschmittel.

German word order is flexible, but this is the most normal, unmarked version here.

Because weil introduces a subordinate clause, and in German subordinate clauses are normally separated from the main clause with a comma.

So:

• Meine Mutter kauft immer dasselbe Waschmittel, weil es gut riecht.

That comma is required in standard German.

Because weil creates a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Compare:

• Main clause: Es riecht gut.
• Subordinate clause: weil es gut riecht

This verb-final pattern is one of the most important German word-order rules to learn.

Because gut is not being used as an adjective directly before a noun. It is being used predicatively with the verb riechen.

So:

• ein gutes Waschmittel = an adjective before a noun, so it gets an ending
• Das Waschmittel riecht gut = no adjective ending

In es gut riecht, gut works like well/good in the expression smells good.

Here riechen means to smell in the sense of to have a smell.

So:

• Es riecht gut = It smells good

Be careful, because riechen can also be used in another way:

• an etwas riechen = to smell/sniff something

Examples:

• Das Parfüm riecht gut. = The perfume smells good.
• Er riecht an der Blume. = He smells/sniffs the flower.

Yes, but the grammar changes.

With denn, the clause stays in normal main-clause word order:

• Meine Mutter kauft immer dasselbe Waschmittel, denn es riecht gut.

With weil, the verb moves to the end:

• Meine Mutter kauft immer dasselbe Waschmittel, weil es gut riecht.

Both can mean because, but weil is often more common in everyday German.

It can suggest both, depending on context.

In this sentence, immer means that whenever your mother buys detergent, she buys the same one. In natural English, that is often expressed as always.

So the idea is:

• she consistently buys the same detergent
• she does this every time she buys detergent

Both ideas fit the sentence well.