I read before I go to sleep.
Leo antes dormirme. (Perfect)
I am good at playing the guitar.
Bueno en tocar la guitarra. (Soy bueno tocando la guitarra) (Be careful not to confuse the "tocando" gerund with the "playing" in English. In English when you use the "ing" form of a verb you can change the verb into a noun. this does not happen in Spanish. So the sentence "I like dancing" would NOT be "me gusta bailando" in Spanish, but "me gusta bailar". When a verb is converted into a noun in Spanish it remains in the infinitive form)
My head is under the table.
Mi cabeza es bajo de la tabla. (Mi cabeza está bajo la mesa) ("ser" is permanent, "estar" is temporary) ("tabla" means "a piece of wood")
Come with me.
Venga conmigo. (perfect)
They are against the government.
Ellos contra el gobierno. (ellos están contra el gobierno)
The dog is behind me.
El perro esta detras me. (el perro está detrás de mí / el perro está detrás mío)
We slept during class.
Dormimos durante los clases. (dormimos durante la clase) ("clase" is femenine, and your sentence in English is singular)
Lots of people were talking about the earthquakes.
Muchos gente hablaron sobre los terremotos. (Mucha gente estaba hablando sobre los terremotos) ("gente" is masculine, and it is a group, thus, singular)
Because of him, I have to wait in line.
A causa de él, tengo esperar en linea. (perfect... only línea, graphic accent on the "i")