How do you say I have a crush on you/her/him/them (you in its tú / Usted forms) ?
If informal you could go with estar por
Estoy por ti / Estoy por él o por ella.
You could also use estar loco por (or chiflado) and me gusta the same way you use crazy for or like somebody in English. Other informal forms would be estar colado por alguien
Estoy colado por ti.
You could also say that somebody te pone if he or she makes you feel aroused
Esa chica me pone. Es guapísima.
Or that you are prendido de
Estoy prendido de esa chica desde que la vi por primera vez.
The way of saying it formally would be saying that you like somebody or you are in love with them or attracted to them.
Estoy enamorado de ti.
Me siento atraído por ti.
In México we say:
For I have a crush on him/her:
Me llama la atención [name here]
this is kinda vague, the beginning, your fellings are not much clear, or you don have enough courage to accept it. Like the first thoughts. You can also say:
but it can be understood as sexual atraccion.
This one, depending on what you're saying means that you like him/her or you just have a crush but again is I have a crush on him/her:
Me gusta [name here]
I have a crush on them, this is unclear for me but it would be:
Me llaman la atención
And the formal and informal thing, I don't thing there's any. If you are saying that to someone you are not going to be formal I mean, there are more informal forms of saying that but I think that It's better not to use them. They're not bad but I think that being informal on this has to be very special, an especific context, especific person, specific mutual history, you can realx and say what you want but, again, it is a special way of doing it.
An example, you know a girl, you have been like, "I like you..." "Me too" but only in thoughts. And then you finally have the courage and ou are very close, so you can say [This is an example, it can be very different depending the people]:
Ya pa que nos andamos con rodeos, te gusto y me gustas, ¡andemos chingao!
Let's stop going around, you like me and I like you, ¡Let´s be a couple god damn it!
Ok, kinda hard to translate. I want to add that "Andar" is being girlfriend and boyfriend.
Mira, Juan y María. ¿Andan? Sí, hace dos días él se le declaró y ahora son novios.
And, declararse is the Would you like to be my girlfriend?.
To have a crush on someone is way, way different than me atrae, me gusta, me agrada, etc. Estar prendido de comes much closer to it. It's a form of enamoramiento to be sure, but it's more and it's different; it's at once more intense and less reliable than being in love. Being in love is more likely to last than having a crush; and you can be in love without losing your head, whereas having a crush can be quite unrealistic. For example, kids have a crush on their teacher, eleven-year-old girls have a crush on Justin Bieber, I had a crush on Ingrid Bergman a few years ago, never mind that if she were still alive she would have beeen like 100 years old, etc.
Although I'm not a native Spanish speaker, I think it's possible that nothing closer than estar enamorado exists, unless estar prendido qualifies, though I suspect that the latter is closer to "being taken with". In a sufficiently clarifying context, it should be easy to discern whether an enamorado has a crush on someone or is in love with them. If the object of their affection is unavailable to them, then there's no two ways about it, it's a crush. Un corazonazo irracional, ilógico y poco razonable might be a good definition, but a real translation would of course have to be much more concise.
The whole time I was writing the above, I was racking my brain, isn't there some Spanish word for this? And, you guessed it, almost as soon as I hit the post button it came to me (and it even has an English cognate!). The word is infatuación.
Infatuación en algunos textos sobre Psicología alude específicamente a cierto estado emocional caracterizado por el dejarse llevar por una pasión irracional, especialmente por el amor adictivo. La infatuación ocurre normalmente al inicio de una relación amorosa. La misma está caracterizada por: urgencia, intensidad, deseo sexual y/o ansiedad, donde hay una extrema absorción del uno con el otro. Se le asocia comúnmente con la juventud y denota infantilismo. Este uso no es muy común y no figura en la edición de 2001 del diccionario de la Real Academia ni en el Diccionario del español actual de Seco, Andrés y Ramos (1999).
The best answer is from Diego. And actually I could say "colado" it's probably the best for "crush", becouse you don't love actually, you only have this "like" feelings.
Nobody say it before,its another way to say it too in Spain. like crush is somewhat..informal, isn´t it? This can be mean "crush" too:
Estoy colgado de él.
The littleraly translation could be I'm hanging with him/her.
You like that person, but you don't really know him/her so much.
But these words look so informal.(colgado/colado)
I'd say if you want to actually say that to your actually crush, say
"Me siento atraído por ti"
"Estoy colgado por él/ Estoy colado por él"
would be like more saying to your mates or friends that you like someone you don't know so much.
It depends on the kind of situation you want to make.
For example if you want to tell this person you want to have a serious relationship you should use Quiero ser tu novio in this case you are saying I want to be your boyfriend.
But if you want to keep the thing easy and keep dating you should use Quisiera que siguiéramos saliendo, para conocernos más in that case you are saying I want to date you to keep knowing each other
Hope it helps you!