Learning the Phrase in Context
The most direct answer depends on whether you're talking about the subject itself or the scheduled class period. In Spain, math is matemáticas, and a math class is clase de matemáticas. In most of Latin America, people say matemáticas as well, but you'll also hear clase de mates used casually among students. Clase de matemáticas is the standard phrase. Pronounce it roughly as "klah-say deh mah-teh-mah-teh-kas." The plural matters here — Spanish treats the subject as a plural noun even when referring to it as a singular concept, so you'll see articles and adjectives agree in the plural form. I learned this the hard way during my first semester abroad. I wrote "la clase de matemática" on a homework assignment and got marked down not because it was unintelligible, but because every teacher in the country immediately noticed the error. It's a small thing, but it signals that you haven't actually lived with the language. The singular matemática does exist in some regional dialects and in certain academic contexts, but for a general class reference, the plural is what you want.
There's a nuance most learners miss. If you're referring to the subject area rather than the physical class, you drop the clase de entirely. So Tengo matemáticas a las diez means "I have math at ten," which is the most common way students actually talk about their schedules. The clase de framing sounds slightly more formal or explanatory, like you're describing the concept to someone who doesn't know the school system. Regional variation is worth noting. In Mexico, you might hear calculismo used in older textbooks to refer specifically to arithmetic, though this is rare in casual conversation. In Argentina and Uruguay, the colloquial mates is extremely common among teenagers and young adults, so you'll hear clase de mates far more often than the full form in those regions. Using the full matemáticas there isn't wrong, but it can sound stiff in informal settings. If you're studying for a test or filling out a form that requires the formal term, always use matemáticas. If you're trying to sound natural talking to a native speaker, clase de mates is your safest bet in most urban Latin American contexts. Just be aware that in very formal academic writing, even mates might be considered too informal, and the full phrase would be expected.
