"I study with my friend on Sundays."
Because it is a compound verb made by adding a noun to the verb します - this is very common in Japanese. 勉強します - to study, そうじ します - to clean, りょうり します - to cook, けんぶつ します - to sightsee or to go sightseeing. を is not used because べんきょう is not the object of the verb - it's part of the verb. Noun + する = compound verb.
Some things to think about whether を should be used:
1) Not all nouns can become "suru verbs". If you look on jisho.org, you'll see that "benkyou" is listed as a "suru verb", meaning 勉強 (benkyou) is a noun meaning "study" and 勉強します (benkyou shimasu) is a verb meaning "study". The word "tenisu" is not listed as a "suru" verb, so テニス (tenisu) is a word that means "tennis", but in Japanese you can't just make "tenisu" into a verb, you have to say "do tennis (play tennis)", テニスをします (tenisu o shimasu), where "tenisu" is the object of the verb and needs to be marked by を.
2) You can say 勉強します (benkyou shimasu) or 勉強をします (benkyou o shimasu), but the grammar and nuance is slightly different. 勉強します is a verb meaning "study". 勉強をします has "benkyou" as the object of the verb "shimasu", making the meaning "do studying".
3) People drop particles in speech all the time, even though it is not grammatically correct.
Just a note that we can say 買い物する (jisho.org is not correct). https://dictionary.goo.ne.jp/jn/37086/meaning/m0u/%E8%B2%B7%E3%81%84%E7%89%A9/
Basically most of the action nouns can be used as [object]+を+[noun]+する or [object]+の+[noun]+をする if it is transtive
Thank you for pointing that out. I'm a bit confused, though, because I thought kaimono was a suru verb and brought it up in another thread, and another user looked it up in their dictionary and said it wasn't listed as a suru verb.
hmm 買い物する sounds natural enough to me so I am sure the usage is ok. I tried to find out an example where the noun + する is natural to me but the dictionaries say otherwise. An example is デュエル (duel) where I find people say デュエルする (mostly with all sorts of card games). On the other hand for the word テニス (tennis), I feel unnatural to say テニスする (The goo dictionary does not say it can, so it is good). Well I think we can stick to any dictionary but just bear in mind it is more conservative that what people are using.
My issue is that everything sounds okay to me without を because I'm so used to native speakers dropping particles, so I'm really trying to understand the correct usage. I wonder if 買い物 didn't used to be a する verb but has become one...
Sports are a great example of non-suru nouns, so I'll edit my original post to avoid confusion, thanks for all your help!
Same problem here. I guess it's just unnatural to say the 私の referring to your own friend, as people will guess you don't learn with THEIR friend although you don't know them. It's more likely to add a (someone)の when you're not talking about your own friends ore stuff.
Anyway, I think Duo should accept it, as it is not grammatically incorrect - and when you're hovering over the "my", it still makes you think that you should add 私の (as the Japanese lessons here are still not developed well enough to differ translation of words in different context, as you can see with the kanji pronunciation).
を (instead of と) here would be incorrect. を follows the direct object of the verb (ie. in this case it would be what you are studying), however there is no direct object in this sentence - the sentence does not state what you are studying with your friends, just that you study with friends on Sunday. If you were to replace と with を you would make your friends the direct object of the verb and the sentence would mean I study my friend(s) on Sundays. と follows nouns to show whom you are performing an action WITH - in this case, the speaker states that they study WITH friends on Sundays. Please also note my and KeithWong's comments above in response to a query with a similar mistake.
Duo didn't accept your sentence because they have a specific sentence and specific grammatical constructions/words in mind that they are trying to teach with that sentence in mind. This is very common with Duo - the other day someone asked why instead of changing the ending of a verb so that it meant 'and' and could join two sentences together Duo wouldn't accept joining two sentences together using そして. I responded that likely it is because Duo is trying to teach us that you can change the endings of verbs so that they act like a conjunction and can be used to join sentences together, whereas if Duo wanted to teach how to join sentences together using actually conjunctions then their sentence would be acceptable. Make sense?
と following a noun - specifically a noun that is a living creature - human/animal indicates that you are carrying out a certain action WITH that noun. For example if you were walking with your dog then と would follow dog to indicate that you were walking with your dog - 犬と 散歩します, or if you were talking on the phone with your mum - 毎日母と でんわで 話します - I talk on the phone with my mum every day - here と follows 母 to indicate that 'mum' is the person that you are talking with. Also, scroll up and have a look through all the comments above, と and its function in this sentence has been queried and discussed at length in the above comments.
Can someone tell me what's the difference between hiragana characters よ and ょ (the last one is a bit smaller). I keep getting failures because I don't have the smaller hiragana yo character in my sentence. I'm so confused. Shouldn't there be just one よcharacter and that's all?
-masu form can be the future tense, or it can be the present tense when talking about a habit. The -te form means that you are doing something right now.
勉強します。 (benkyou shimasu)
[future] I will study.
[present] I study (as a regular habit).
勉強しています。(benkyou shite imasu)
[present progressive] I am studying (right now).
Have you read the previous comments? https://forum.duolingo.com/comment/23250371?comment_id=32261680