Just as with any dictionary, not all possible translations are the best, or the correct, translation. Our job, as learners, is to make the effort to determine, learn the standard, or correct, translations.
"Escritorio" could be translated as "office.'
However, "desk" is the more common, and the more obvious/standard translation.
I just focus on the basic, standard meanings, and do not try to use more obscure, less common, meanings. I also use this site a lot to help me understand words: http://www.spanishdict.com/translate/escritorio http://www.spanishdict.com/translate/office
The verb ‘sentar [a alguien]’ is transitive, meaning “to seat [someone]”. Spanish does not have a stative verb meaning “to sit”, so it is expressed paraphrastically as ‘estar sentado’ = “to be seated”.
The present participle ‘sentando’ means “in the process of seating [someone]”, so «Él se está sentando en su escritorio.» or «Él está sentandose en su escritorio.» means “He is in the process of sitting down at his desk.”, literally “He is seating himself at his desk.”.
I see the "sentado" is a past participle used as a modifier, describing the state of being seated or sitting. The guy is seated/sitting at the desk. "Esta sentando" would mean he is the process of sitting down at the desk, because the progressive tenses are for emphasizing action that is actually occurring at that very moment. In that case, sentar ( he is seating himself) would be used reflexively (sentarse) and you'd need to add a "se." Anyway, that's my take on it.
Past participles do describe an past action, but it doesn't have to be completed, it can be, but there's also room for the action still going on. Anyway, past participles are also used as adjectives, when they're used that way, they describe a state/condition of something, especially when they are used with "estar" which is used to describe a state/condition. Therefore by using "estar" with the past participle "sentado" the state of being seated/the state of sitting is being described. While "está sentando" is the present progressive, which uses the gerund "sitting" and describes an action that is going on right now (he is sitting -right at this moment-). Basically what it comes down it is whether you're describing the persons state (being seated) or an action that is going on at that very moment (sitting). However, if you want to stay close to the original sentence which uses a past participle (he is seated), "está sentado" would be more appropriate.
Whilst, coincidentally, 'siento' is the same in the first person singular for both meanings, they do in fact derive from two different verbs, Sentar (to sit) and Sentir (to feel). In the present tense these conjugate as:- Sentar: siento, sientas, sienta, sentamos, sentáis, sientan Sentir: siento, sientes, siente, sentimos, sentís, sienten, There are many words in English which, too, have completely different meanings - eg 'pen' - a writing implement or an enclosure for holding animals
Grandmompam, see rspring's parsing of present progressive for what Duolingo says is the correct translations. But, your translation is technically correct because the past participle, "sentado" means "seated." Time of his seating seems to be the issue, and rspring makes a case for this occurring in the present. Duolingo, IMHO, should accept your answer and/or give more context for "time."
I translated this as 'He is sat at his desk'. The OED gives 'sat' as the pp of 'to sit'. DL marked as incorrect. 'He is sat' is unambiguous (he sat down of his own volition. 'He is seated' can either mean the same or that he was shown to his seat by someone else. Either way, DL was wrong to mark 'He is sat at his desk' as incorrect.
The verb “to sit” is intransitive. English does not have passives of intransitive verbs. And if it did, the agent would be omitted or at least demoted to a “by…” prepositional phrase, to something like *“At his desk was sat [by him]”.
The corresponding transitive verb for human objects is the causative “to seat”, the past participle of which is “seated”, not “sat”, yielding the passive “He is seated at his desk.”, not *“He is sat at his desk.”.
I think everyone's right in a way:
"To sit" is an intranstive verb, where the person performs the action.
"To seat" is a transitive verb, where the action is performed on the person i.e. the person is given a seat.
If you say "he has sat at his desk" is using the present perfect, meaning he has performed this action at an unspecified point in the past.
"A man is sat at his desk", although perhaps not officially correct grammatically, is the way it is often said in British English as a colloquialism. I would argue that "to sit" is being used here as a transitive verb, as in the man has made himself sit down, and is now sat at his desk. Grammar in language does evolve!