When you have just named two things, use the former for the first one and the latter for the second, instead of repeating the nouns. Two limits matter: the pair works only for exactly two items (with three, write the last or name it), and the reader has to still hold both in mind, so keep them close — across a paragraph break the reference fades. A contrast word such as while or whereas sharpens the comparison, but it is not required.
The former and the latter
the former /ðə ˈfɔrmər/ — The study compared online and in-person learning; the former offered flexibility, while the latter provided direct interaction.
the latter /ðə ˈlætər/ — Both candidates were strong, but the latter had more relevant experience.
whereas /wɛrˈæz/ — The report weighed cost and quality; the former mattered to finance, whereas the latter mattered to design.
Not ""The study examined stress, burnout and absence. The latter is most common in education." (three things were named, so 'the latter' has no second item to point to)" — say ""The study examined stress, burnout and absence. The last of these is most common in education."". The former / the latter works only when you have named exactly two things. With three or more, use the last or name the item.
Try it: The report mentioned cost and quality. ______ mattered more to design. (The latter / The former / Such symptoms) Answer: The latter — "The latter" points to the second of the two items named, which is quality, and quality is what design cares about. "The former" would point back to cost, the first item, so it names the wrong thing here. "Such symptoms" refers back to a category of examples and has nothing to fill in a two-item contrast.
This + summary noun
To point back to a whole idea you just described, you can attach a summary noun to this: this decline, this trend, this issue. For research results, these findings packages the results into one tidy reference. A bare this is perfectly good English when only one thing it could refer to has just been mentioned ('This suggests that the effect is not linear'). Reach for the summary noun when several ideas compete for the reference, or — the real payoff — when you want the noun to label the idea: the same drop in numbers becomes this decline, this adjustment or this collapse depending on how you want the reader to judge it.
this decline /ðɪs dɪˈklaɪn/ — Enrollment dropped by 15 percent in a single semester; this decline prompted an urgent review by the board.
this trend /ðɪs trɛnd/ — The pilot attracted fewer learners each month, and this trend concerned the coordinators.
these findings /ðiz ˈfaɪndɪŋz/ — The researchers identified three recurring patterns; these findings suggest a broader problem.
Not ""Staffing was cut and the software was replaced. This was a problem for the organization." (two changes were just named, so 'this' could point to either)" — say ""Staffing was cut and the software was replaced. This upheaval was a problem for the organization."". A bare this is fine when only one idea could be meant. Add a summary noun when several compete — and choose the noun for the judgment it carries.
Try it: Enrollment dropped by 15 percent in a single semester. ______ prompted an urgent review by the board. (This decline / The former / Whereby) Answer: This decline — "This decline" packages the drop in enrollment into one precise summary noun, which is exactly what a formal back-reference needs. "The former" requires a pair of items to choose between, but only one idea was mentioned. "Whereby" means 'by which' and introduces a method or process, so it cannot stand as the subject of this sentence.
Try it: They adopted a policy ______ students could defer exams in serious emergencies. (whereby / the latter / these findings) Answer: whereby — "Whereby" means 'by which' or 'through which' and introduces the procedure the policy sets up, so it fits after 'a policy' perfectly. "The latter" points back to the second of two named items, and none were named here. "These findings" refers to research results, which is unrelated to a policy mechanism.