"only" can be used as a substitute for "merely" only when you're downplaying what you've got- "It's only a scratch," and "it's merely a scratch" mean almost the same thing and can be used interchangeably, while "it's only a peanut butter sandwich" and "it's merely a peanut butter sandwich" are further apart in meaning.

They imply two different things and are used for different purposes.

Only is an exclusivity signal: He only likes pizza. You are separating the concept "pizza" from the set of "everything else" He only likes pizza, he doesn't like anything else. We're focusing on "Pizza" in this sentence.

Merely is a value differential: He MERELY likes pizza. Merely implies that one is settling for one thing when other potential values exist: He merely "likes" pizza, he doesn't love it. Merely in this sentence points to "likes." He merely likes pizza. We aren't talking about the pizza, but about his feelings for it.

So they indicate two different things. If you only have class on Tuesday and Thursday, you are separating those days from your free days in the week.

If you merely have class on Tuesdays and Thursdays, then you are indicating that either you feel you should have class on more days, or that you have something better you could be doing- either way, those days, or the classes on them, are not what they could be.

Think of it this way: "Only" makes a box and then talks about what's in that box- Only likes pizza, only has class these days... "Merely" makes a box and compares it to what's outside...Merely likes, merely has class.. "Could love pizza if.." "these days aren't my videogame days."
Source(s):
http://dictionary.sensagent.com/merely/e…