Some English words appear to be similar but they have different meanings. Here are some of them.
Imply / infer
‘Imply’ means to suggest that something is true without saying this directly. 
‘Infer’ is to form an opinion that something is probably true because of information that you have.
A lot can be inferred from the statistics.
Impromptu / extempore
Both words mean ‘spontaneous’ and ‘unpremeditated.’ However, ‘extempore’ is customarily reserved to describe an off-the-cuff speech.
The band gave an impromptu concert.
Rex delivered an extempore speech.
Improve / ameliorate
‘Improve’ means to make something better or to become better.
The course is ideal for students to improve their English.
‘Ameliorate’ means to make a bad situation better or less harmful.
It is not clear what can be done to ameliorate the situation.
Inapt / inept
An inapt phrase or statement is not right for a particular situation.
Leo made a very inapt comment at the inaugural sessions.
‘Inept’ means not good at doing something.
Roy was criticised for his inept handling of the crisis.
Incapable / unable
‘Incapable’ means not able to do something.
Sam seemed incapable of understanding how she felt.
‘Unable’ means not able to do something.
George was unable to find out what had happened.
Although ‘incapable’ and ‘unable’ have similar meanings, ‘unable’ implies an inability to do something at a particular time or because of some special circumstances.
Incredible / incredulous
‘Incredible’ means extremely good, large, or great.
There was blood everywhere and the pain was incredible.
‘Incredulous’ means unable or unwilling to believe something.
Sara shot him an incredulous look.
Inculcate / imbue
‘Inculcate’ means to fix ideas or principles in someone’s mind.
As a father I try to inculcate a sense of responsibility in my children.
‘Imbue’ means to make someone or something have a quality, idea, or emotion very strongly.
His philosophical writings are imbued with religious belief.
Index / indexes / indices
‘Index’ means an alphabetical list of names or subjects at the back of a book with numbers of the pages where they can be found. The plural form ‘indexes’ means a set of cards or a database containing information usually arranged in alphabetical order and used especially in a library. The plural form ‘indices’ means a standard on which the level of something can be judged or measured.
Indoor / Indoors
‘Indoor’ means used or happening inside a building.
The Smiths have an indoor swimming pool.
‘Indoors’ means into or inside a building
Let’s go indoors and have something to eat.
‘Indoor’ is an adjective and ‘indoors’ is an adverb.
Outdoor / outdoors
‘Outdoor’ means existing, happening or used outside a building.
Jim leads a healthy outdoor life.
‘Outdoors’ means outside a building
It’s warm enough to eat outdoors tonight.
‘Outdoor’ is an adjective and ‘outdoors’ is an adverb.
Inequity / inequality
‘Inequity’ means lack of fairness.
There are inequities in the distribution of research funding.
‘Inequality’ means an unfair situation in which some groups in society have more money, opportunities or power than others.
There are inequalities in wealth distribution.
Infect / infest
‘Infect’ means to give someone a disease.
People with the Coronavirus may feel perfectly well, but they can still infect others.
If insects or rats infest a place, there are a lot of them and they usually cause damage.
Nancy’s kitchen is infested with cockroaches.