Stop, Avoid, Prevent

And now to something a little different.

The use of STOP, AVOID, and PREVENT is often misunderstood by non-native speakers of English

Sentences like these are all too common;
  - We closed the doors to AVOID them from leaving. (STOP or PREVENT)
  - A rock was placed behind the wheel of the car to AVOID it from moving (STOP)

In Portuguese the most common words used are impedir, deter and evitar. Parar can be understood to mean STOP but is sometimes used to mean PREVENT (usually impedir) and vice versa, where impedir is used to mean STOP. Evitar seems to be translated more as AVOID when it might be better translated as PREVENT. As in English, a lot of these uses are inconsistent, depend a lot on context and DO lead to confusion.

The most obvious misuse is when the speaker wants to say STOP, but chooses AVOID instead.
Normally when correcting this I give the following example.

Flooding in low lying regions is common when there is a lot of rain.
   - To PREVENT flooding, it is essential to keep the drains clean and clear of blockages. A better drainage system is also recommended. (STOP, but meaning before the actual event, not during that event.)
   - To AVOID flooding, it is suggested that you take a different route, one where the roads are higher and less subject to flooding. (GO AROUND)

                                                         --ooOO\\//OOoo--

AVOID is most often used to refer to the act of taking a different path, or course of action, because an obstacle is PREVENTING further progress, or we might even want to PREVENT further progress.
   - When we see an object on the road while driving, most often we steer the car away from the object to AVOID hitting it.
   - In the office, we sometimes get on an elevator so as to AVOID talking to someone we do not want to talk to.
   - In a meeting, we try to AVOID talking about difficult situations.

There is nothing in these sentences which would allow us to use anything except AVOID.

                                                         --ooOO\\//OOoo--

We use PREVENT in the same way we use STOP. But there are many special cases where STOP just does not work.

Logic plays a hand here. When we know that something can be PREVENTED, its not always the case that it can be STOPPED.

Take HIV for example. We can take steps to PREVENT (reduce) the spread of the Virus, but we can not effectively STOP the spread. We take PREVENTATIVE measures, like using Condoms and practicing safe sex for example.

We use security measures to PREVENT crime. It does not always STOP crime, but it makes crime harder to commit.

We can STOP a car by applying the brakes (Not PREVENT).
We can STOP a fight by separating the people fighting.
We can PREVENT a fight (from starting), usually only before a fight can start.

A flood can be STOPPED if there are enough people and there is enough equipment to do so. That usually happens during the flood and means keeping the flood from spreading. Compare that to PREVENTING a flood (SEE above) and AVOIDING a flood (SEE above).

                                                         --ooOO\\//OOoo--

While in these examples the differences can be made to seem clear, the dictionary definitions can lead to confusion.  In the OALD dictionary I use and recommend, I found the following definitions.
STOP means to no longer move, to not continue.
PREVENT means STOP.
AVOID means PREVENT.
Obviously, AVOID cannot mean to no longer move, nor not continue.

The examples in the dictionary go a long way, however, to try to qualify the simplified definitions and need to be read and understood to more fully understand the differences.

   - The accident could have been AVOIDED. Means that the accident only happened because steps were NOT taken to PREVENT such a possibility. The driver should have been driving more carefully. The pedestrian should have waited for the traffic lights to change.
   - The accident could have been PREVENTED. Warning signs should have been placed along the road.
   - They narrowly AVOIDED defeat - they were lucky not to lose, or They could not AVOID defeat - Defeat was inevitable.
   - The name was changed to AVOID confusion with another firm. (This gives a sense of PREVENT, except PREVENT has too strong a meaning to be used here.
   - The government took steps to PREVENT  a scandal. (To STOP)
   - We STOPPED for the night in Las Vegas.
   - The car STOPPED at the traffic lights.

                                                         --ooOO\\//OOoo--

(There is a special problem associated with STOP)

In the sentence - I have STOPPED to talk to him, it is usually understood by non-native speakers to mean - I am no longer speaking to him.
It means in fact, that the speaker STOPPED doing something else so as to be able to speak to him. The correct expression is  -  I have STOPPED speaking to him.

I have STOPPED to smoke - Should be I have STOPPED smoking.
I have STOPPED to drink - Should be I have STOPPED drinking. (usually Alcohol)

Let`s STOP to eat means of course, Let`s STOP driving or working to begin eating.

I hope that this goes some way to explaining when and how you should use STOP, AVOID and PREVENT.