In a sentence, if there are two parts, and the first part begins with an "if"- it is called an if-clause or a conditional sentence. If-clauses are also known as conditional sentences.
Conditional sentences have a condition in them - "if" something is done/happens - only then the second part of the sentences is complete.
1. If you study well, you will get the first rank.
2. If Ravi tells me the truth, I will let him go.
3. If Helen comes early, we will start the class.
4. If it rained, you would get wet. 
5. If people complained, things would get better.
Check the usage of modal verbs - "will and would":  
  • The modal verb "will/will not" is used in the case of the simple present tense, and if the condition is very likely to happen.
  • The modal verb "would/would not" is used in case of simple past tense, and if the condition is only probable or unlikely to happen.
Sometimes, two sentences can be combined with an "if-clause" as follows:
1. Study well. You will get first rank!
If you study well, you will get the first rank!
2. Don't be lazy. You will regret it later.
If you are lazy, you will regret it later.
3. Don't tease animals. They will get frightened.
If you tease animals, they will get frightened.
It is also important to note the usage of the following:
  • "if you want to" - for positive sentences.
  • "if you don’t want to" - for negative sentences.
  • "if you want him/her to" for third person sentences.
1. You can use my laptop if you want to.
2. Don't eat outside food, if you don't want to.
3. He'll complete your work if you want him to.
  • Use negative (don't want to) if you see negative in the sentence (Don't)
  • Use the third person (if you want him/her) if you see a third person(He/She) in the sentence.
If-clauses used in the lesson:
  1. Amma says Paati will leave for our Paddu Mama’s house if she knows we have a cat.
  2. She wouldn’t be so upset if she knew about the poor beggar.
  3. And Amma, if they did fit, would you really not have minded?