Recurring prose problems at FAC, Part 1:
- but: But should usually separate two opposing statements.
- could: Many sentences that I run across containing "could" are ambivalent or contain an opinion that needs to be attributed per WP:WEASEL.
- however: Opinions on the best use (or avoidance) of "however" are evolving, in every flavor of English. Generally my style is to figure out if the sentence can work without a contrasting word, and if it can't, then I try to use some word other than "however" unless I really want to emphasize the contrast (per Chicago 5.206 and 5.207). Chicago recommends that you make an effort to put however somewhere that seems least likely to require commas before it or after it, but sometimes a comma or two is necessary.
- Quotations require in-text attribution, usually at the end of the sentence, per WP:CITE#When quoting someone and Chicago Ch. 13.
- Semicolons: per Chicago 6.58, avoid semicolons to separate the elements of a series ("X; Y; and Z") if possible, but use them if the elements contain commas and rewording can't fix the ambiguity.
- would: In a narrative, usually avoid "would", except for the "future-in-past" tense, when you are briefly mentioning something that happens after the events in your narrative. From Battle of Magersfontein: "The poor maps and lack of reconnaissance would prove critical to the outcome of the battle." (The writer mentions a future event to help the reader understand the importance of the maps, then resumes the narrative.) Since "would" is often used for events that actually happened (in the future relative to the narrative), it can be confusing to use "would" in the main clause to signify planned or predicted events that never happened; "were to" or "was to" is often a better choice for these.