Translation:I do not want a shirt because I want a skirt.
I'll give an insight into this topic, in Spanish you can see the 4 possible combinations:
- Por qué
- Por que
being each one different:
- Porqué: noun. Always preceeded by article (el, un) or some kind of determiner (su, otro, ...). Means cause, reason. E.g. Él nunca explica el porqué de sus decisiones - He never explains the reason of his decisions.
- Porque: cause conjunction. Preceeds a subordinated sentence which gives an explanation. Means because. E.g. Lo entiendo porque lo explicas muy bien. - I understand it because you explain it very well.
- Por qué: only in interrogative sentences, both direct or indirect. Por is preposition, qué is interrogative pronoun. Means why. E.g. ¿Por qué no vienes? (direct) - Why don't you come? - No entiendo por qué se comporta así (indirect) - I don't understand why he behaves like that.
- Por que: formed by preposition por and relative pronoun que, it can be substituted by el cual, la cual. Preceeds a subordinated sentence. E.g. Se esforzó por que consiguiera aprobar el examen - He made an effort to pass the exam.
I hope this helps a lot. Even native speakers don't use the correct form and even think theirs is the right one.
Can be but not always. Observe these examples of the last case:
Estas son las razones por que he querido venir. == Estas son las razones por las cuales he querido venir == These are the reasons for me to come [which made me want to come].
que can be subjuntive clause while por is mandatory due to the specific verb requiring it:
- Inclinarse por (tend to have an opinion or a preference over alternatives): Me inclino por que no vendrá.
The "lo" is not a subject pronoun like "yo" and "tú" are. It is a masculine/neutral direct object: him or it (and object pronouns go before verbs in Spanish). For example, in the sentence: "I read it" the 'it' is a direct object. So in Spanish, you would write it like: Yo lo leo. or simply Lo leo. (because the yo is implied and not needed).
So in the example sentence given: Lo entiendo porque lo explicas muy bien. - I understand it because you explain it very well.
The lo does not mean I, yo would still be the correct subject to go there but is left out because it is unnecessary. Instead, the lo is the direct object of "understand"...understand it. And the same is true for the lo before explicas.: you explain it...
The sentence could also be written: Yo lo entiendo porque tú lo explicas muy bien.
Here are some more examples if I haven't written enough already. :)
You eat it. = Tú lo comes. = Lo comes.
She sees him. = Ella lo ve. = Lo ve.
I drink it. = Yo lo bebo. = Lo bebo.
The English sentence actually doesn't need a comma because the second part, "because I want a skirt," is not an independent clause and "because" is not a coordinating conjunction. The deal with the because-sentences is that when you have the because-clause first, you need the comma after it, but when it's second, you don't use the comma.
It has always seemed to me that the Spanish grammar is very similar, so it's probably so with this sentence at least.
The independent clause rule you refer to doesn't apply to "because I want a skirt;" rather it applies to that which follows the subordinating conjunction. That is, "I want a skirt." And that IS an independent clause. So, if you are using that rule as a proof of appropriate punctuation, then you need the comma.
That said, arguments have been made for omitting the comma in situations like these in order to provide a particular semantic emphasis. In this case, omitting the comma stresses the following meaning: "The reason I don't want a shirt is that I want a skirt."
For you grammar geeks out there, the reason for this nuance interpretation is that the absence of a comma stresses an equal absence of apposition, and therefore the direct relationship between wanting one and not wanting another.
For you grammar geeks and non-grammar geeks both, common sense tells you that the sentence provided by duolingo makes no sense in either language (boo, duolingo), because the desire for one thing does not automatically exclude the desire for another, so the causative relationship indicated is not logical.
But it is grammatical. Either with or without the comma. Pick your poison.
In another version of this question an acceptable answer is "I do not want a shirt, because I would like a skirt." Then in this version "I do not want a shirt, as I would like a skirt," is unacceptable.
If one of those is acceptable, then both are, although I would personally say that neither should be.