Contrast: but, however, although

WordUseExample
but joining two contrasting ideas in one sentence (coordinating) I like coffee, but I don't drink it at night.
however linking two separate sentences; introduces a contrast (start new sentence) The hotel was expensive. However, the room was beautiful.
although / even though before a clause; introduces the surprising/unexpected part Although it was raining, we went for a walk. · I went out although I was tired.
However vs Although: "However" needs a full stop (or semicolon) before it. "Although" goes inside one sentence. ✓ "It was cold. However, she wore a t-shirt." ✓ "Although it was cold, she wore a t-shirt." ✗ "However it was cold, she wore a t-shirt."

Reason and result: because, so, as

WordUseExample
because gives the reason (answers "why?") I stayed home because I was ill. · Because it was late, we left.
so gives the result (= therefore) I was tired, so I went to bed early.
as / since reason at the start of a sentence (slightly formal) As it was getting late, we decided to leave.
because vs so: "I didn't go out BECAUSE it was raining." (rain = reason) · "It was raining, SO I didn't go out." (not going out = result). One introduces the cause, the other introduces the effect.

Time connectors: when, before, after, until, as soon as

WordExample
whenWhen I arrived, she was already there. · Call me when you get home.
beforeBrush your teeth before you go to bed. · I finished work before she called.
afterAfter I had dinner, I watched TV. · She called me after the meeting.
until / tillWait until I get back. · I worked until midnight.
as soon asAs soon as I get home, I'll call you. (= immediately when)

Exercises

Choose the correct connector.

1. I missed the bus, ___ I was late for work.

2. I was late for work ___ I missed the bus.

3. The film was very long. ___, it was very entertaining.

4. ___ she was tired, she kept working until 11pm.

5. Don't leave ___ I get back — I need to talk to you.