As an aid to coherence, you should avoid unnecessary shifts in point of view.
Avoid unnecessary shifts in subject.
From Caribou Ridge we looked down, and before us lay a beautiful valley. (awkward)
From Caribou Ridge we looked down and saw a beautiful valley. (improved)
I am studying chemistry, although laboratory work does not especially appeal to me. (awkward) I am studying chemistry, although I do not especially like laboratory work. (improved)
Avoid unnecessary shifts in voice. (A shift in voice nearly always causes also a shift in subject.)
Because Maria wanted to learn how to sew, a course ill home economics was taken by her. (awkward — Wanted is in active voice; was taken is in passive voice.)
Because Maria wanted to learn how to sew, she took a course in home economics. (correct—Both verbs are in active voice.)
Avoid unnecessary shifts in tense.
Philip pulls over a chair and sat down, (not consistent—Pulls is present tense; sat is past tense.)
Philip pulled over a chair and sat down, (consistent— Both verbs are in the past tense.)
Before he leaves the room, he checked the lights. (Not consistent — Leaves is present tense; checked is past tense.)
Before he leaves the room, he checks the lights. (consistent—both verbs are in the present tense.)
Before he left the room, he checked the lights, (consistent—both verbs are in the past tense.)
Note. Of course a shift in tense is necessary whenever there is a change in time.
He wrote a manual, which is still used today.
Avoid unnecessary shifts in mood.
If he were in difficulty and was unable to overcome it, would you not help him? (shift from subjunctive indicative)
If he were in difficulty and were unable to overcome it, would you not help him? (correct)
Avoid unnecessary shifts in person.
If one really studies hard and learns how to work, you can succeed in America. (shift from third person td second person)
If one really studies hard and learns how to work, he can succeed in America. (correct)
We volunteered to go on the mission, realizing that might never return, (shift from first person to third person).
We volunteered to go on the mission, realizing that we might never return, (correct)
Avoid unnecessary shifts in number.
It is rewarding for women to be a nurse. (shift from plural to singular)
It is rewarding for women to be nurses. (consistent)
A cat is supposedly one of the most enjoyable pets to own, but they bring with them certain disadvantages. (shift from singular to plural)
A cat is supposedly one of the most enjoyable pets to own, but he brings with him certain disadvantages. (consistent)
Avoid unnecessary shifts from indirect to direct discourse.
Tim wondered whether he would make the team and will he get to play much, (shift from indirect to direct discourse)
Tim wondered whether he would make the team and whether he would get to play much, (correct— indirect discourse)
Tim wondered, “Will I make the team? Will I get to play much?” (correct—direct discourse)
Avoid unintentional shifts in style or tone within a sentence or within a larger piece of writing. Do not shift from formal to informal or from informal to formal.
The process of election affords a moral certainty that the office of President will never fall to the lot of any man who doesn’t have his act together, (inappropriate shift from formal to informal style)
The process of election affords a moral certainty, that the office of President will never fall to the lot of any man who is not in an eminent degree endowed with the requisite qualifications. — Alexander Hamilton (appropriate—formal style used throughout)
