“H.H. Holmes was the first American that became a serial killer.” H.H. Holmes (an alias for Herman Mudgett) was a person, not an object. It’s an insult, even to a serial killer, to call him “that.” The sentence should read, “H.H. Holmes was the first American who became a serial killer.”
2. Who or Whom?
Who is doing what to whom? When in doubt, ask yourself that simple question. The one who performs the action–the subject–is who. The one who experiences the action–the object–is whom.
3. I see this alot. It’s wrong.
“A lot” is always two words. “There were a lot of problems with the manuscript.”
4. There, Their and They’re
Homophones create another common error. It’s pretty simple, really. “There” indicates a location. “Their” is possessive. “They’re” is the contraction of “they are.” “They’re over there, examining the storm damage to their house.”
5. Subject-Verb Agreement
The subject and verb must always agree on the math. Singular subjects must have singular verbs.
“The killer screams at his victim.”
“The killers scream at their victim.”
“I am not a convicted felon.”
“We are not convicted felons.”