Shreya G.

The most straightforward answer to your question is that "understanding" is a passive skill, but "speaking" is an active skill. Understanding only requires passive or uncontrolled memory, but speaking requires controlled memory. You have probably been in a situation where you need to go somewhere you had been once before. You can't remember the name of the street until you see the name on a street sign, then you remember it. That's an uncontrolled memory. You know something when it is presented to you, but you can't recall it on your own. That is also the difference between understanding a language and speaking it.
However, I would also add that if you can understand English, you can even speak English. You just can't speak it as "well" as you would like to. Learning to speak it means practising and practising means you are going to make mistakes. You will never make a mistake if you quietly listen to someone else speaking English, but if you want to improve your ability to speak, you have to accept making mistakes as part of the process.

Practice speaking with your friends, practice speaking in front of the mirror. Learn new words and start using them in your daily vocabulary. The more you talk, the more comfortable you will get with the language. And do not try to speak by converting the Hindi sentence into English that will always make you hesitate.