There are a series of English words which are frequently confused by students.
This article aims to identify them and correct those mistakes.  

1. beside vs. besides

Beside means "next to".
Besides means "in addition (to)", "also", "moreover"
e.g. You can sit down beside me if you want.
e.g. I don't like that TV programme. Besides, I don't have time to watch it.    

2. chair vs. seat  

A chair is a moveable seat for one person.
A seat is a place to sit in a cinema, train, bus, etc.  
e.g. There are four chairs around the kitchen table.  
e.g. When I arrived in the compartment someone was sitting in my seat.  

3. classic vs. classical

Classic means "best", "first-class" or "masterly".
Classical means "old and established" or "traditional".
e.g. His most recent novel is a classic.
e.g. I don't like classical music very much.  

4. cook vs. cooker .  

A cook is a person who cooks.
A cooker is a machine for cooking food.  
e.g. Jennifer is training to be a cook.  
e.g. You should clean the cooker at least once a week.  

5. control vs. inspect

To control means "to have power over", "make s.o. or sth. do what you want".
To inspect means "to check", "examine" or "verify".
e.g. Will the police be able to control the hooligans?
e.g. The ticket-officer inspected the passengers' tickets.  

6. dish vs. plate  

Both dish and plate can refer to round ceramic containers for food. Food is often brought to the table in a dish from which people serve their individual plates. However, when plates (and cups, etc.) are dirty they are called "the dishes". But when they are clean they are called "plates". We use dish for a specific combination of food.
e.g. Could you do the dishes and them put the clean plates away?  
e.g. We need some more plates, could you fetch them?  
e.g. Paella is my favourite dish.  

7. economic vs. economical

Economic means "relating to the economy".
Economical means "cheap", "inexpensive".
e.g. His thesis was about long-term economic growth. 
e.g. I think you'll find out products very economical.  

8. efficient vs. effective  

Efficient means "quick and "without waste".
Effective means "producing the result that you want"  
e.g. This machine is very efficient, so you'll have a higher production in less time with a lower electricity bill.
e.g. These pills are the most effective against headaches, though they take slightly longer to take effect.

9. floor vs. ground

The floor is what you walk on inside a building.
The ground is what you walk on outside a building.
e.g. Come indoors and clean the floor, will you?
e.g. In the morning the ground was frozen solid.  

10. hardly vs. hard  

Hard as an adjective can mean rigid", "strict", "difficult" or arduous". Hard as an adverb can mean strictly" or "arduously".
Hardly is an adverb with means almost not", "only just"  
e.g. We worked hard for three months and finally passed the exam.  
e.g. I can hardly hear you.  

11. historic vs. historical  

Historic means "important in history", "extraordinary", "famous".
Historical means "connected with history" or "real", "authentic, "documented".
e.g. In a historic vote, the government has decided to abolish capital punishment.
e.g. The municipal council should do more to preserve historic buildings.  
e.g. She writes historical novels.  
e.g. There are no historical facts to support his assertions.

12. homework vs. housework  

Homework is work which teachers ask students to do at home.
Housework is the different cleaning activities at home.  
e.g. The teacher wanted to know why they hadn't done their homework.  
e.g. Husbands should do part of the housework.  

13. job vs. work  

Job is countable. Work is uncountable.
e.g. I'm looking for a job.  
e.g. I’m looking for work.  

14. official vs. officer  

An officer is a person with a special rank or authority in the army, navy, air force, police force or customs service. An official is a person with authority in the (local or national) government or in a trade union.  
e.g. My brother is a naval officer, my sister is an official on the town council.  

15. package vs. packet  

A package is a parcel, usually sent by post. A packet is a commercially prepared box, bag or container (especially of food) which is sold in a shop.
e.g. A package has just arrive for you. Open it up and let's see what it is!  
e.g. He bought a packet of cigarettes and two packets of chewing gum.  

16. personal vs. personnel  

Personal is an adjective meaning "private", "individual".
Personnel is a noun meaning "staff", "employees".
e.g. He's having a lot of personal problems at the moment.  
e.g. The company's having a lot of personnel problems; they'll need to find new staff if they're going to meet their deadlines.

17. sofa vs. armchair  

A sofa (or "settee") is a long comfortable seat for more than one person.
An armchair is a large comfortable seat for just one person.  
e.g. A three-piece suite consists of a sofa and two armchairs.  

18. steal vs. rob  

To steal means to take sth. illegally.
To rob means either to take sth. illegally, take from s.o. illegally, or take from somewhere illegally.
e.g. She stole the jewellery.
e.g. She robbed the jewellery.
e.g. She robbed the jeweller.
e.g. She robbed the jeweller's shop  

19. steps vs. stairs  

Stairs are (or a staircase is) a flight/flights of steps inside a buil­ding from one floor to the next. Outside a building we have to talk about steps.
e.g. There is a majestic flight of steps outside the palace.  
e.g. Go through the hall, up the stairs and it's the first door on your right.  

20. stranger vs. foreigner  

A stranger is a person who you have never met before.
A foreigner is a person from another country (who could be a close friend).  
e.g. You should teach your children not to talk to strangers.  
e.g. You can't tell my husband is a foreigner from his physical appearance.

Glossary
pill: tablet, solid medicine
slightly: a little
frozen: solid because of temperatures
capital punishment: the death penalty
local council: town council, "government" of a town.
assertion: declaration, claim, statement
army: military force trained to fight on land  
customs service: authority which controls the importation of products.
trade union: organisation of workers
parcel: object(s) covered in paper so that it/they can be sent by post.
packet (UK English) - pack (US English)
to meet one's deadlines: satisfy the time limits agreed with clients.
three-piece suite (UK English) - living room suite (US English) 
flight (of steps/stairs): collection, series