Why? / Because

We use because before a clause (a sentence subject verb and complement). Use because of when you put the noun after the reason.

Examples:

• I went to sleep because I was tired.
  [Use because and NOT because of >> it is followed by a sentence I (Subject) was (verb) tired (complement)]
• I had to go early to the office because of the meeting
  [Use because of >> it is followed by a noun (meeting)]

- Maria left the party because she was feeling sick.
- Carlos couldn't play because of his injury (lesion).
- I didn't pass the test because I didn't study.
- I feel so sad because of his death.

NOTA: Se usa el because y because of para expresar razones. Muchas veces la traducción es porque pero también puede ser por. Cuando es "por" tienen que tener cuidado de no usar el "for". Veamos este ejemplo,

No pude dormir por el ruido.

En este caso se entiende que la razón por la cual no durmió fue a causa de la lluvia. Entonces es una razón y no se usa el "for" sino el because of. Se usa este y no es "because" porque no le sigue una oración sino solo un sustantivo.

I couldn't sleep because of the rain.

Recuerden que se utiliza el "because" cuando a este le siga una oración.

I didn't dance because I was sad. >>>> No pude bailar porque estaba cansado.

Acá se utiliza el "because" pues le sigue la oración (clause) I was sad.

So

- I was tired so I went to bed.
- My favorite soccer team was playing bad so they lost.
- I couldn't sleep so I took some pills.
- There wasn't any food so we went out to eat.

But / Although

Use but and although to show contrast. Although can be used at the beginning of the sentence

Examples:

- Juan studied all night. He didn't pass the test. (There is contrast)
- Juan studied all night but he didn't pass the test.
- Although Juan studied all night, he didn't pass the test.

• I went to the party but didn't dance.
• Although it was very cold, we had a great time.
• My father went to the supermarket but it was closed.