Nocivo means harmful, injurious, damaging. Here is an example sentence: El tabaco es nocivo por el alquitrán y la nicotina que contiene. (Tobacco is harmful due to the tar and nicotine that it contains.)
You can take the test to see if you can spell the 15 Spanish words that many people often misspell at La Ortografía Infinita.
to complicate matters, to make things difficult, to split hairs, to nitpick
Of course, literally, this phrase means to look for three feet on a cat. But what one really intends to express with this idiomatic expression (modismo) is: to complicate things or to look for trouble. It is easy to locate a four legged cat, but three legged cats are more difficult to come by! To use some English idioms as a definition for this expression you could say: make a mountain out of a molehill or to go over something with a fine tooth comb.
No hay que buscarle tres pies al gato. (No need to go looking for trouble.)
I found this infographic from nextlingo on Pinterest. Ya and todavía can be very confusing. Ya can be used to say now, yet and already. Todavía can be used to say yet and still. As you can see, ya is used to indicate a change from the past to the present and todavía indicates continuity from the past to the present.
Here are a few more examples.
Ya viven en nuevo piso. (They’re living in a new apartment now.)
Ya ha olvidado lo que hiciste. (She has already forgotten what you did.)
¿Han llamado ya? (Have they called yet?)
No lo quiere todavía. (He doesn’t want it yet.)
Llevo trabajando todo el día y todavía no he terminado. (I’ve been working all afternoon and still have not finished.)
Ya and todavía have other meanings and uses, but this visual has helped me to begin to understand how to use these 2 adverbs. I hope it is helpful to you as well!
Estoyhasta las narices de tu desorden. (I am fed up with your mess.)
Practica Español has a list of 78 Spanish expressions using body parts. In addition to hasta las narices, you can find ir con pies de plomo (to tread carefully), traer de cabeza (to drive someone mad), no tener pelos en la lengua (not to mince words), sacar los ojos (to quarrel), tener el agua al cuello (to be up to one’s neck in it), and venir como anillo al dedo (to be just right).