From what I've learned in Duolingo so far, the simple present in French ("Je marche.") is about equal to the simple present and present continuous tenses in English ("I walk" vs. "I am walking").
In English, the simple present tense can be used to describe usual or repeated activities (http://www.englishpage.com/verbpage/simplepresent.html). So "Je marche le matin" could translate to "I am walking in the morning." or "I walk in the morning." the latter of which essentially equates to "I walk every morning." or "I walk most mornings."
To convey the idea of "every morning" (repeated action/habit), you can say "tous les matins" or "chaque matin" or simply "le matin". All mean the same and are frequently used.
This also works with other moments of the day (l'après-midi, le soir, la nuit) and days of the week (le lundi, le mardi, etc.)
Your sentence ("I am walking in the morning.") describes what the speaker is doing at that moment: It is morning and she is walking. The French sentence ("Je marche le matin.") states that the speaker often walks in the mornings. It may not even be morning when she says it. So it is more accurately translated as, "I (often/usually) walk in the morning."
I don't quite understand the grammatical order of this sentence... The way it is written looks like "le matin" is the direct object of "marche" (what I guess is not correct, right?). Why isn't there a preposition before "le matin" or intead of "le"? Something like "Je marche à matin" or "Je marche dans le matin"...
When I hear "I work mornings" (very common as you pointed out), I think it's implied that you work all morning or most of the morning. So hearing "I walk mornings," I think maybe it sounds strange because there is a similar implication and it would be unusual to walk all or most of the morning.
According to Sitesurf (see reply to .tortue above), the construction implies habitual action in French. I agree with you that the same word order has a further connotation in English.
In fact, it is very close to "I work the morning shift." So "mornings" really is behaving like a direct object, which is what Diogo.Alvarez said. I just thought it might be helpful to people to see a (superficially) similar construction in English.
"le matin", "le soir", "l'après-midi", "le soir", "le vendredi" are used to date an event.
"marcher" does not have a direct object (ex: we don't walk the dog, nous promenons le chien).
The notion of "in the morning (during the morning)" refers to a duration in French:
"dans la matinée", "dans la soirée", "dans l'après-midi", "dans la semaine", "dans la journée de vendredi"
Thank you all for the answers, they were very helpful. So let me see if I got this: "Je marche le matin" is implying a habit like "I (usually) walk in the mornings", and "Je marche dans la matinée" is something like "I spend my morning walking" (completely different structure, I know, just used it to be clear about the idea of the sentence). Am I correct? (please, say I am... hahaha)
"Marcher" is simply "to walk", whereas "promener" (or) "se promener" has to do with taking (sb/sth) for a walk or to go for a walk yourself. So "marcher" is just saying "walk" as opposed to some other means of transportation. "Promener" is about having a stroll (i.e., no destination in mind and you come back to where you started).
Your sentence is correct English, but it doesn't have the right meaning. Your sentence is about what you are currently doing or what you intend to do the next day. The French, "Je marche le matin" is about habitual action. So "I (usually/always) walk in the morning" is a better translation.
See Sitesurf and my comments on this page.
It is just that English requires a preposition in this construction:
"I walk in the morning." or "I walk during the morning."
Alternatively, you can leave out the word "the," and then the preposition is not needed:
"I walk every morning." or even just "I walk mornings."
All of these are reasonable translations of the French "Je march le matin."
"I am walking in the morning" was marked incorrect. Is that appropriate?
From all/most other verb examples so far we've seen that the Noun + verb (I eat) and noun + am + verb + ing (I am eating) are both acceptable from the present tense conjugation in French, is that not correct in this instance?
As I understand it, the expression "le matin" (or "le lundi" or "le soir" or whatever) specifically denotes an habitual action. It is unusual in English to use the progressive tense in this construction, although I'm sure I could concoct a circumstance in which it would work.
"I walk in the morning" carries the same habitual implication that the French sentence does. It is a clear and simple translation.
"I am walking in the morning," without further context, could mean the same thing, but most readers would assume you were describing a single event as it was happening.
Note: I am not sure how you would describe such a single event in French. Maybe, "Je marche dans la matinée", but maybe you'd have to be more specific: "En ce moment, je marche dans la matinée," or "Je suis en train de marcher dans la matinée". It all sounds quite odd to me.
Matin is one of those words that can refer to a point in time (division) or it can refer to a period of time (duration).
Division morning is matin. Duration morning is matinée.
I walk in the morning = matin because it is about a point in time. I walk all morning is matinée because it is about a period of time.
Expect to see the feminine form when using duration constructions.
I wrote 'I am walking in the morning' which was marked wrong but is in fact perfectly correct. I f the question was 'are you walking tomorrow morning or tomorrow afternoon', the answer would be as I wrote. This should be accepted as a correct answer. Also 'je + verb' translates as 'I + act' or 'I am + acting' in every context.