At & In : I am at my mom's place in London.. Means I am at a place which is smaller than London. At & In : I am at London in England.. Again London is smaller than England.. It's a helping verb which assists in comparison or differentiation. In: We are in a Hotel having dinner... Again IN is a helping verb to describe usually & mostly in present tenses and also with other tenses On: I am on my way to London.. Means going or in process . On: I am on my to meet you. This word ON is used for describing the process of our work or meets.