If you mean the word retard, its first meaning in English is the same as in French : "To delay, to hold back", and it actually comes from French as well. It took 400 years after the word was introduced into the English language before it was used to describe children who were slow (or late) to develop mental habilities (actually retarded, which became retard in a very informal and offensive way even later).
If you go to this site: http://www.acapela-group.com/ you can choose from several French voices to hear some variations in pronunciation of "tu es en retard". You'll notice there is a strong tendency to more or less swallow the first syllable of "retard", but it is there. "...en tard" would sound noticeably different, at least to an accustomed ear.
You will also notice, by the way, that none of the voices makes an elision between "es" and "en", although our RoboGirl does. You should not hear the "s" of "es", even when the following word begins with a vowel.
DianaM: Are you sure the Duo bot is wrong? This site http://ielanguages.com/blog/french-and-its-secret-liaisons/ says that liaisons are optional after all forms of être except est (where it's obligatory).
You're right, it is optional. And thanks for your very interesting link. Here's another with a somewhat different perspective: http://french.about.com/library/pronunciation/bl-liaisons-o.htm
Note that according to this (usually very helpful) site, liaison after a verb (other than "est") is "very high register" - i.e., optional but rarely heard. The last time I cited it, though, Sitesurf said that in the particular case we were discussing, she preferred to use the liaison. So there's that - ha.
Personally, I find liaison with "es" or "as" sounds funny. For what that's worth.
Merci aussi Diana! I always bow to your wisdom (bow-wow to your puppy too!) and had some trepidation about posting my question. My background in French is purely academic (& except for my past year with Duo very old) so like Santa checking his list, I checked and rechecked that page to make sure I was getting it right. Thanks for taking the time to post your insights to Duo. It's only through reading the comments (which I do religiously) that I've learned anything here (I mostly use the app). For me, the comments ARE the lesson.
As a French native, I can assure you I make that liaison, and not just when I want to sound formal. It is optional and I've heard people not making it (it doesn't sound weird either way).
I can't cite a rule saying it is right or wrong, it's just how natural it sounds to me. Though when I'm not sure, and I want to give correct info to learners, I usually quote the French.about.com article that DianaM linked to.
Merci Bastou for weighing in. I've always been surprised but most grateful that native speakers bother to take the time to set things right here. Believe me, Duo would be like a bird with its wings clipped without you and others like you chipping (cheeping?) in. Perhaps I should go enroll in English and do likewise.
Perhaps I should go enroll in English and do likewise.
You certainly should! There are way fewer people helping learners in the forums for courses taught from other languages than English and, as you've noticed yourself, help from natives is always appreciated.
As to why we do it, besides the happiness of feeling helpful, we usually learn as much about both languages when we help people studying one of them (I also help people learning my third language from my second, and so on with various combinations). And although I don't learn as much about English and French (my first two languages in which I'm quite fluent), I still do every once in a while, even in French. Some things that are a certain way because that's the way it is suddenly get a very understandable explanation when I have to seek it out to help learners.
Indeed, as DianaM said, "tu es retard" is not grammatically correct. "tu es en retard" means both of what you proposed.
You could also say "tu as été retardé(e)" to mean "you have been delayed (by some external event)", but be careful of the verb tense, because "tu es retardé(e)" (mentally implied) is the exact French equivalent of "you're a retard".
"Retard" is a French noun, meaning "lateness." "En retard" is an adverb, meaning "late." While "en" has no real translation here, you can say that it means "in a state of" so "en retard," would then mean "in a state of lateness".
By itself, "en" can have many meanings and uses. Two of the most common are as a preposition (as in "en classe" for "in class"), and as an object pronoun (as in "J'en ai un" or "I have one.").
retard is a noun meaning delay and expresses behind schedule or overdue
en retard is an adverbial phrase meaning late (not punctual) and must be used with a verb, usually être or arriver
tard is an adverb and refers to lateness and means late
It is only possible to say tard if you can ask a question replacing the word late with what time?. hier soir, tard – yesterday evening, late. Therefore You cannot say "Il est tard", for he is late, because the question what time is he* does not make sense. Il est tard means it is late