Contrast: but, however, although, whereas

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.
whereas directly compares two facts side by side — neither is surprising, just different My brother loves sports, whereas I prefer reading.
However vs Although vs Whereas: "However" needs a full stop (or semicolon) before it. "Although" introduces something unexpected inside one sentence. "Whereas" simply compares two different facts, with no surprise involved — a neutral contrast. ✓ "It was cold. However, she wore a t-shirt." ✓ "Although it was cold, she wore a t-shirt." ✓ "I like tea, whereas she prefers coffee." (just a difference, nothing unexpected)

Reason: because, as / since

WordUseExample
because gives the reason (answers "why?") I stayed home because I was ill. · Because it was late, we left.
as / since reason at the start of a sentence (slightly formal) As it was getting late, we decided to leave.
Already know "so"? "So" (the result connector — "I was tired, so I went to bed early") is covered together with the other FANBOYS connectors — see the link in the sidebar if you need a refresher.

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. My sister loves the city, ___ I prefer the countryside.

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

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

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

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