Fíjate

The key idea behind fijarse is noticing details. Let's look at some examples.

• No sé si usted se habrá fijado, pero ... I don't know if you have noticed, but ....
• Si se fija uno, sí se nota que ... If you look closely, you'll notice that ....

Sometimes fijarse is about checking something over for correctness or acceptability: Fíjate bien en las fechas. Check the dates carefully to make sure you are available then. Or it can be more about what are interested in: Yo me fijé en el fotógrafo. The photographer caught my eye (but not yours). Me fijo más en la cara que en el cuerpo. I'm more into someone's face than their body. I tend to notice faces more than bodies.

¡Fíjate!

As a stand-alone utterance, ¡Fíjate! means Watch where you're going! The longer version is: Fíjate por donde caminas. I heard Esquincle, ¡fíjate! in a movie, meaning: Watch where you're going, you almost crashed into me, running like that, you brat.

— ¡Fíjate, hombre! Watch where you're going! (Someone just bumped into the speaker)
— ¡Fíjate, tú! You watch where you're going! (a rude reply)

If there's a difference between Fíjate and ¡Aguas!, it would be that fíjate is more specifically a warning about where you are going.

Fíjate can be tagged on at the end of an utterance to add sassiness or emphasis.

• No, muchas gracias pero no quiero, fíjate. Thanks but no thanks. For your informmation, I'm not interested.
• Pues él no es así, fíjate. As a matter of fact, he's not at all like that. He's not the way you say he is. He's much better than that.

Fíjate que

Fíjate que works more generally as a way of either setting the listener straight about something or prefacing some bit of information or news.

• Pues sí. Fíjate que sí. As a matter of fact, yes.
• Fíjate que no. It may surprise you to learn this, but no, I am not from France. In response to something like Eres de Francia, ¿verdad?
• Ay joven, fíjate que no está. I'm sorry, the person you are looking for is not here right now.

As you can see from the final example, fíjate que ... can serve as a polite, although possibly uncaring, way of giving bad news. In this sense, it is similar to con la pena.