Being able to switch between tenses is a good way to illustrate your skill with the language.
A word of warning
Changing tenses can prove your English skill to an examiner. However, using tenses incorrectly will result in losing marks.
Also, switching tenses at the wrong time or too often can make an essay unreadable. Only change tenses inside a sentence if you are sure it will still make sense.
Twelve tenses
It is worth knowing the twelve basic tenses
(note: ‘will’ can be swapped for ‘be going to’. See here for the difference.)
Simple
Present Simple – I think there are too many cars on the road.
Past Simple – The first cars appeared in the early 20th century and became popular after the war.
Future Simple – Car ownership will continue to grow.
Continuous (be …ing)
Present Continuous – Developing nations are buying automobiles in large numbers.
Past Continuous – When the first cars came into production most people were using horse-drawn vehicles, trains, or walking.
Future Continuous – By the year 2050 the majority of the world’s population will be driving.
Perfect (have (done))
Present Perfect – As more families have bought two vehicles cities have become gridlocked.
Past Perfect – In the 2000s Beijing tried to control car numbers but the damage the 1980s and 1990s had done was irreversible.
Future Perfect – The average person currently living will have driven about half a million miles before he/she dies.
Perfect Continuous (have been …ing)
Present Perfect Continuous – People have been driving for only a century, but the car has changed our landscape.
Past Perfect Continuous – Before the combustion engine was invented populations had been moving around cities on foot or horse for several hundred years.
Future Perfect Continuous – Without a change in attitude it is possible that by the time fossil fuels run out people will have been depending on their vehicles to such an extent many will find adaptation impossible.
Ideas for different tenses
Some tenses – such as the past perfect, past perfect continuous, and future perfect continuous – are difficult to use, and even harder to use in an essay. But some tenses are easier.
(Note: these are only ideas for ways to switch tenses)
Opinions – present tense – I believe that people must force the change and buy alternative-energy fueled cars.
Quoting facts – present tense – There are more cars on the road now than at any other time in history.
Evidence/ facts (past) – past tense – In the 1950s most middle-class families only owned one car; now they have two.
Evidence (recent) – present perfect – Recently some cities have introduced congestion charges for driving in the city centre.
Current trends – present perfect continuous – Recently more people have been looking at buying electric cars, and inventors have been toying with ideas such as magnet-powered ignition systems.
Future fact – future tense – If the rate of population growth and car ownership continues more and more cities will face the same problems.
Future projections (now vs. future time) – future continuous – At current rates it is inevitable that within the next century more people will be suffering from pollution-related diseases, more nations will be involved in energy-related conflicts, and more families will be struggling to pay for fuel than now or ever before.