Please don't use "sth" on this forum.
Student or Learner
What's the difference between "suck sth", "suck at sth" and "suck on sth"?
***** NOT A TEACHER *****
1. "Plants suck moisture from the earth." -- Random House Webster's Unabridged Dictionary.
2. "The child sucked on her thumb." -- The New Oxford American Dictionary.
3. I suck at math. (That is absolutely true. I know nothing about math. So I hate it, and I fear it!)
P.S. Do you mind my giving you some respectful advice? Even in 2014, some people still find that word as used in #3 as somewhat vulgar -- especially when used in this sense:
Mona: What do you think about Mr. X's class?
James: It sucks! (It's very bad.)
In the business world, it might be a good idea NOT to use that word -- except in the senses of #1 and #2.