An Overview of Conjunctions:

Conjunctions are used to link two similar grammatical structures. For example, y links two phrases of equal grammatical status, like two noun phrases (mi hermano y su amigo), while que links two verb phrases in a subordinate relationship (yo veo que tú vienes).

Here are some points to keep in mind when writing with conjunctions:

y/e:

• means "and" and joins two phrases of equal grammatical status.
• when this conjunction precedes a word that begins with "i" or "hi", it must change form to e, e.g.: Fueron al campo e hicieron una gran fiesta. "They went to the country and had a big party."
• when using a conjoined structure like mi amigo y yo ..., the following verb should be in the nosotros form, e.g.: Mi amigo y yo llegamos tarde. "My friend and I arrive late."

o/u:

• means "or" and joins two phrases of equal grammatical status.
• when this conjunction precedes a word that begins with "o" or "ho", it must change form to u, e.g.: Hablaremos con mujeres u hombres. "We will talk with women or men."

pero:

• means "but" and joins two phrases of equal grammatical status
• what follows pero can be emphasized with sí, e.g.: Él no tiene mucho dinero pero sí sabe hacer muchas cosas. "He doesn't have a lot of money but he does know how to do a lot of things."

sino:

• means "but rather" and joins two phrases of equal grammatical status.
• sino is distinct and NOT interchangeable with pero
• occurs in statements involving negation and a correction, e.g.: No quiero cerveza, sino vino. "I don't want beer, but rather wine."
• use sino que before a verb phrase, e.g.: No caminaba, sino que corría a la casa. "He didn't walk, but rather ran home."
• note that si no, written as two words, means "otherwise".

que:

• means "that" and is used to link two clauses (phrases with conjugated verbs) in a subordinate relationship (a subordinate clause embedded in a main clause by the conjunction)
• unlike English where the conjunction "that" is often optional, the conjunction que is required in Spanish to link to a second clause, e.g.: Yo pienso que es importante hablar claramente con la familia. "I think (that) it is important to speak clearly with one's family."

si:

• means "if" and is used to link two clauses where the main clause is conditional upon the subordinate clause occurring.
• do NOT write this conjunction with an accent; sí with an accent is a different word that means "yes"
• do NOT use the conditional verb form or the present subjunctive directly after si (use the imperfect, the present, the future, or the imperfect subjunctive instead) (see more on the present conditional and conditional perfect verb forms here), e.g.: Iré al mercado si tengo tiempo. "I will go to the market if I have time." Si tuviera tiempo, iría al mercado. "If I had time, I would go to the market." (hypothetical) Si tenía tiempo, iba al mercado. "If I had time, I would go to the market." (habitual)

porque:

• means "because" and links two clauses in a subordinate relationship, the subordinate clause after porque being the reason for the main clause
• e.g.: No voy a ir porque no quiero. "I'm not going to go because I don't want to."
• note that por qué, written as two words and with an accent means "why".

aunque:

• means "although/even if".
• the clause after aunque often appears in the subjunctive if the event has or had not yet occurred, e.g.: No lo haría aunque me ruegue. "I wouldn't do it even if he begs me."