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

Mitbewohnerin is the feminine form and means female flatmate / roommate.

• der Mitbewohner = a male flatmate
• die Mitbewohnerin = a female flatmate

The ending -in is a very common way to make a noun specifically feminine in German.

Meine means my, and its ending has to agree with the noun that follows it.

Here, Mitbewohnerin is:

• feminine
• singular
• in the nominative case because it is the subject of the sentence

So the correct form is meine.

Compare:

• mein Mitbewohner = my male flatmate
• meine Mitbewohnerin = my female flatmate
• mein Waschmittel = my detergent

It is in the accusative case, because it is the direct object of kauft.

Ask yourself: What is my flatmate buying?
Answer: dasselbe Waschmittel

Since Waschmittel is neuter singular, the accusative form looks the same as the nominative here:

• nominative: dasselbe Waschmittel
• accusative: dasselbe Waschmittel

So there is no visible change in this sentence.

Dasselbe means the same.

So dasselbe Waschmittel means the same detergent.

A useful point: German often writes this as one word when it is used like this:

• dasselbe Buch = the same book
• dieselbe Idee = the same idea
• derselbe Mann = the same man

It changes form depending on the gender of the noun:

• derselbe for masculine
• dieselbe for feminine
• dasselbe for neuter

Since Waschmittel is neuter, we use dasselbe.

This is a very common learner question.

In everyday German, many speakers use dasselbe and das gleiche almost like synonyms, but traditionally there is a difference:

• dasselbe = the very same exact item
• das gleiche = an identical item of the same kind

For example:

• Wir benutzen dasselbe Buch. = We are using the very same physical book.
• Wir benutzen das gleiche Buch. = We are using the same edition/type of book, but not necessarily the exact same copy.

In your sentence, dasselbe Waschmittel suggests she keeps buying the same exact product/brand/type.

Immer is an adverb meaning always, and in a normal main clause it often appears after the conjugated verb and before the object.

The sentence structure is:

• Meine Mitbewohnerin = subject
• kauft = conjugated verb
• immer = adverb
• dasselbe Waschmittel = object

So:

Meine Mitbewohnerin kauft immer dasselbe Waschmittel.

This is a very natural word order in German.

Because weil means because, and it introduces a subordinate clause. In German subordinate clauses, the conjugated verb usually goes to the end.

So:

• main clause: Meine Mitbewohnerin kauft immer dasselbe Waschmittel
• subordinate clause: weil es gut riecht

Notice how riecht comes at the end of the weil clause.

This is one of the most important German word-order rules.

Compare:

• Es riecht gut. = It smells good.
• ..., weil es gut riecht. = ..., because it smells good.

Es refers to das Waschmittel.

Since Waschmittel is a neuter noun, the pronoun used for it is es.

So:

• das Waschmittel → es

In English we might also say it:

• because it smells good

Because gut here is an adverb, not an adjective describing a noun.

It modifies the verb riechen:

• gut riechen = to smell good

So there is no adjective ending.

Compare:

• ein gutes Waschmittel = a good detergent
  (gutes describes the noun Waschmittel)
• Das Waschmittel riecht gut. = The detergent smells good.
  (gut describes how it smells)

In German, subordinate clauses are normally separated from the main clause by a comma.

Since weil es gut riecht is a subordinate clause, you need a comma before it:

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

This is required in standard written German.

Yes, to some extent.

For example, you can put the weil clause first:

Weil es gut riecht, kauft meine Mitbewohnerin immer dasselbe Waschmittel.

Notice what happens in the main clause after that:

• the verb kauft must still stay in the second position
• so meine Mitbewohnerin moves after the verb

This is a classic German word-order pattern: when something else comes first, the verb still stays second.

Yes, very similarly.

Riechen can mean:

• to smell / have a smell
• sometimes also to smell something with your nose, depending on context

In this sentence, es gut riecht means it smells good.

Other examples:

• Die Blumen riechen gut. = The flowers smell good.
• Das riecht seltsam. = That smells strange.
• Ich rieche Rauch. = I smell smoke.

So German uses riechen in a way that is often close to English smell.