however, therefore, furthermore (linking ideas) (B2 Grammar)

In formal writing, however links two ideas that pull against each other. The second idea is a surprise or the opposite of what the first one led you to expect. It's the formal cousin of "but" (which you'd use in speech). Form: it starts a new sentence with a comma after it — ...hard. However, he failed. — or follows a semicolon — ...hard; however, he failed. Don't glue the two ideas together with only a comma.

however: the other side

• He worked very hard. However, he didn't pass the exam.
• The plan is expensive. However, it is the safest option.
• Sales were strong. However, profits stayed flat.
• Not "He worked very hard, however he didn't pass the exam." — say "He worked very hard. However, he didn't pass the exam.". A plain comma before however is a comma splice. Use a full stop or a semicolon before it, and a comma after it.
• Try it: Amina's report says the new software is powerful. The next thing she wants to say is the catch: most of the staff find it confusing. Which linking word fits the idea she's adding? (The software is powerful. However, most staff find it confusing. / The software is powerful. Therefore, most staff find it confusing.) Answer: The software is powerful. However, most staff find it confusing. — The second idea pulls against the first (powerful, yet confusing), so it needs however. "Therefore" would mean being powerful causes the confusion, which isn't what she means. She's marking the catch, not a result.

therefore: because of that

therefore links a cause to its result. The second idea is what follows from the first. It's the formal cousin of "so"; other words with this job: thus, consequently, as a result. Form: like however, it opens a new sentence with a comma after it — ...costs. Therefore, we... — or follows a semicolon. A comma alone before it is a comma splice.

• Sales dropped sharply. Therefore, we must cut costs.
• The data was incomplete. Therefore, the study was paused.
• Demand has risen. Therefore, prices are going up.
• Not "Sales dropped sharply, therefore we must cut costs." — say "Sales dropped sharply. Therefore, we must cut costs.". Don't join the cause and its result with only a comma. Put a full stop or semicolon before therefore, and a comma after.
• Try it: Lars writes that the bridge was found to be unsafe. The next idea is the action that follows from that: the council closed it. Which linking word fits the idea that follows? (The bridge was unsafe. Therefore, the council closed it. / The bridge was unsafe. Furthermore, the council closed it.) Answer: The bridge was unsafe. Therefore, the council closed it. — Closing the bridge is what follows from it being unsafe, so therefore fits. "Furthermore" would add a separate extra point, but this isn't a new fact, it's the result of the first one.

furthermore: one more point

Furthermore adds a second, separate point that pushes the same idea further. It doesn't contrast and it doesn't show a result — it simply says "and here's more." It's the formal cousin of "and" or "what's more"; moreover and in addition do the same job. Form: it opens a new sentence with a comma after it — ...experience. Furthermore, she... — or follows a semicolon. Don't link the two ideas with only a comma.

• The house is spacious. Furthermore, it is close to good schools.
• The plan cuts costs. Furthermore, it saves time.
• She has ten years' experience. Furthermore, she speaks three languages.
• Not "The house is spacious, furthermore it is close to good schools." — say "The house is spacious. Furthermore, it is close to good schools.". Furthermore starts a new sentence (after a full stop or semicolon) with a comma after it — a plain comma before it is a comma splice.
• Try it: Mei lists a second, separate advantage of the candidate: she also speaks three languages. Mei explains the result that follows from a 40% drop in sales: the team must cut spending. Match each one. Which linking word fits each? Answer: Mei lists a second, separate advantage of the candidate: she also speaks three languages. → 1 → She has experience. Furthermore, she speaks three languages. · 2 → Sales fell 40%. Therefore, we must cut spending. Mei explains the result that follows from a 40% drop in sales: the team must cut spending. → 1 → She has experience. Therefore, she speaks three languages. · 2 → Sales fell 40%. Furthermore, we must cut spending. — A second, separate point takes furthermore: she has experience, furthermore she speaks three languages. A consequence takes therefore: sales fell, therefore we must cut spending. One adds a point; the other shows what follows.

as long as = only if; in case = as a precaution

As long as sets a condition that must be true: it happens only if that stays true. In case is a precaution: you do something now because something might happen later. They are not interchangeable.

• You can borrow the car as long as you fill it up. (the condition)
• Take an umbrella in case it rains. (a precaution, in advance)
• We'll go ahead as long as the budget holds.
• Not "I'll take my phone as long as I need to call someone." — say "I'll take my phone in case I need to call someone.". A precaution for a possible future need is in case, not as long as (which sets a condition).
• Try it: Lars sets a single rule the loan depends on: he will lend the money only if it is paid back by Friday. Sofia takes a precaution before a long drive: she packs water now because the road might be closed later. Match each one. Which linking word fits each? Answer: Lars sets a single rule the loan depends on: he will lend the money only if it is paid back by Friday. → 1 → I'll lend it as long as you repay me by Friday. · 2 → I packed water in case the road is closed. Sofia takes a precaution before a long drive: she packs water now because the road might be closed later. → 1 → I'll lend it in case you repay me by Friday. · 2 → I packed water as long as the road is closed. — A single rule the action depends on takes as long as: I'll lend it as long as you repay me. A precaution taken in advance takes in case: I packed water in case the road is closed. One sets the condition; the other prepares for a maybe.