Page 127 - Practical English Usage 3ed - Michael Swan, Oxford

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but meaning 'only'
In older English, but was used to mean 'only', but this is now very unusual.
She is but a child.
Note: bur is usually pronounced /bat/. not IbAtl (see 616). For except. see 194.
For but as a conjunction and ellipsis after but. see 178.
by: time not later than
By can mean 'not later than'.
I'll be home by five o'clock. (= at or before five)
Can 1 bo"ow your car?- Yes, but 1 must have it back by tonight. (= tonight
or before)
By can also suggest the idea of 'progress up to a particular time'.
By the end o f the meal, everybody was drunk. Before a verb, we use by the time (that).
1'11 be in bed by the time you get home.
By the time that the guards realised what was happening, the gang were
already inside the bank.
For the difference between by and until. see 602.6.
other meanings
By can also be used to talk about time in the rather literary expressions by day and by night (= during the day/night).
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119 1
By means 'just at the side of; something that is by you may be closer than something that is near you. Compare:
We live near the sea. (perhaps five kilometres away) We live by the sea. (We can see it.)
by (method, agent) and with (tools etc) the difference
By and with can both be used to say how somebody does something, but there is an important difference.
We use by to talk about an action - what we do to get a result. We use with to talk about a tool or other object - what we use to get a result. Compare: ~
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by (method, agent) and with (tools etc) 119
He worked by night and slept by day. Note also day by day, hour by hour etc.
The situation is getting more serious day by day. (= '" And one can pay by the hour, by the day etc.
In this job we're paid by the hour.
You can hire a bicycle by the day or by the week.
by and near
each day.)
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