Prepositions and time: BY vs. UNTIL

[For German speakers]
The German word bis can be translated into English as either by or until, which makes it hard to know which to use. Here is the answer.

Short version
Use by when talking about an event that occurs at a single point in time like finishing.

I will finish the project by Friday

Use until when talking about an activity or state that continues over a period of time like working (an activity) or being asleep (a state).

I will work until 10pm
I was asleep until the phone rang

Longer version
Let’s call events that occur at a single point in time ‘point events’, and events and states that continue over a period of time ‘duratives’.

THE RULE
By is used with point events and until is used with duratives.

EXAMPLES

Event or state	Allowed prepositions
POINT EVENTS I finished the project... The show will start...	by, at
DURATIVE EVENTS I worked on the project... The actors performed... DURATIVE STATES I liked the show... I was happy...	from, until, for

Working happens over a period of time so has a beginning (from), an end (until) and a duration (for), which makes it a durative.

A COMMON MISTAKE

WRONG: I will finish the project until Friday.

The duration of a finishing event is zero, which explains why you cannot use the prepositions from, until or for in the above sentence.

RIGHT: I will finish the project by Friday.

This means the finishing event will occur at a point in time before Friday (e.g., Thursday).

Advanced notes
POINT EVENTS AT THE BOUNDARIES OF DURATIVES
Point events are often boundaries in time that indicate the beginning and end of durative states and events. Verbs like start and finish, and begin and end are very general words for these boundaries, but there are also specific words that can be used for particular activities and states. For example, to talk about the boundaries of a durative like sleeping, you would normally use specific phrases like falling asleep and waking up.
A durative like knowing can have different kinds of point events at its beginning, depending on how the knowledge is acquired (by learning, discovery, hearsay, etc.).

I learnt that...
I discovered that...
I heard that...

PREPOSITIONS: SPACE AND TIME
The prepositions by and at can be used to indicate points in space as well as time.

The road passes by the lake.
I stopped at the lake.

The prepositions from and until/till/to can be used to indicate boundaries in space as well as time.

The road leads from London to Chichester.

The preposition for can be used to indicate lengths in space as well as time.

The road goes for 90 km.

See other posts about the relationship between space and time for more discussion.