Learner ends a sentence where it ends, rather than joining independent clauses with a comma or with nothing.
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Six sentences. Two are joined correctly already.
It is expensive, many people cannot afford it. Is the comma enough?
The stand-alone test, three legal joins, and the words that fake it.
Cover the second half. Could it be a sentence alone?
Cover the first half. Same question.
Two yeses means a comma is not strong enough.
A run-on makes the reader hunt for the boundary. That costs marks.
A comma buckling, and three joins that hold.

The same two blocks. One fastening is not built for the load.
Three worked repairs. Test first, then fix.
1. It is expensive, many people cannot afford it.
Can the second half stand alone? So the comma must become . Repaired: It is expensive, many people cannot afford it.
2. Some people disagree, however the evidence is clear.
Is however a joining word? So it needs before it. Repaired: Some people disagree however, the evidence is clear.
3. Although it is expensive, many people buy it.
Can the first half stand alone? So the comma is . Repaired:
Eight sentences of continuous text. Two are already right.
Public transport in large cities is under strain. (1) Fares have risen every year, many commuters have simply stopped travelling. (2) The buses are still crowded they run less often than before. (3) Some councils have cut routes entirely, however the demand has not gone away. (4) Although the subsidy is expensive, most cities keep paying it. (5) Drivers face the same problem the roads are full at every hour. (6) Nothing will change quickly therefore planners are looking at longer timescales. (7) It is costly but necessary. (8) The money has to come from somewhere, otherwise the network will shrink further.
One pair of ideas, punctuated three different ways.
Two ideas that go together — for example, public transport is expensive to run and most cities keep funding it.
Use your own pair, or that one.
Version 1: two separate sentences. Version 2: one sentence, comma plus and, but or so. Version 3: one sentence, joined by a semicolon.
Numbered.