"Je mange" ( no 's') can be translated into English as either "I eat" and or "I'm eating", depending on the context. There is no present progressive in French. The phrase "en train de", as in "Je suis en train de manger", is not a specific tense : it's more of an idiomatic expression which you can use if you particularly want to emphasise the fact that an action is in progress. An English present progressive does not have to be translated with 'en train de'. In most cases, it corresponds to a present simple. For example: Hi. What are you doing? I'm eating. would be translated as: Salut. Qu'est-ce que tu fais (là)? Je mange. There's no need to use the 'en train de' expression, because the progressive nature of the action is obvious from the context.

What Suki said is interesting, but not 100% accurate. You should ignore the whole of what Chris wrote though. Regarding Suki's : indeed the context helps to understand if "je mange" should translate as "I eat" or "I am eating", though, the the context does not always exist / is not always clear. Also, there is indeed a need to use "présent progressif" : when you want to emphasize the fact that the action is happening. Regarding Chris' : you can, of course, use "en train" for most actions, including "fumer une cigarette". Does not matter if the action is short, long (all this is relative), you can use "en train" with "smoking a cigaret" or "building a house". Second point : "en train" does not emphasize being busy. You can say "je suis en train de sortir" which means "I am currently geting out of home", or "je suis en train de marcher" which means "I am walking". Third point : we almost never use "être en train" in present tense in the way you described, as a replacement for past tense. Hope this help. Feel free to drop me a msg if you need further explanations.

There is a related discussion here: https://www.italki.com/question/442178 Briefly, one of teachers comments on a first en train de usage that en train de is quite common and then he comments on a later usage ("il est en train de fumer") and explains that en train de doesn't work for two reasons: (1) en train de adds emphasis and the smoker is not an important element of the story, and (2) en train de is for longer duration actions and smoking is considered a shorter duration action.

There are at least two uses of the "en train de faire" structure. In spoken French, it is used to say that a person is busy doing something (which takes a while) and often to hint that they should not be bothered. - Est-ce que Jean vient avec nous? - Non. Il est en train de réviser parce qu'il va passer un examen lundi. In written French, some stories use the present, sometimes called the narrative present, for completed actions (as a vivid replacement for the passé simple or passé composé). The être en train structure is a way to describe an incomplete action. Un jour, Jean se lève, s'habille et va au bureau. Il arrive en retard. Il est en train de lire ses courriels quand le grand patron lui téléphone.