Translation:I enter the hospital.
Depends on the verb - it wouldn't work with hairu but it would with say sampo suru or shinjiru. For example kooen wo sampo shimasu - I walk through the park. Kooen ni sampo shimasu - two possibilities - I walk to the park or I walk IN the park. Also anata wo shinjimasu - I believe you. anata ni shinjimasu - I believe IN you. Also certain verbs take specific particles as a rule/set construction.
はいり (hairi). You know by studying kanji and learning and memorising their different readings and also by context. はいります - means to enter, while いります means to need. Aside from having different kanji you should be able to figure out from context which is which - for instance, in this sentence いります would not make sense especially with the particle に showing motion towards or into. Books, apps, dictionaries, reading Japanese books and writing in Japanese regularly (eg. I often write notes in Japanese, shopping lists, reminders of appointments on the whiteboard on the fridge and chat with nihonjin friends on messenger and skype amongst other things) will all help you to learn and retain what you have learnt.
入り口 (いりぐち) and 入ります (はいります) - entrance and to enter respectfully, use the same kanji with different pronunciations (commonly called "readings" ie. ways of reading them). Then there is also another verb いります (要ります) - note the different kanji - which means to need. All kanji have multiple ways of reading them (a few examples - 正しい (正しい) - tadashii - correct/true, 証（あかし）-akashi - testimony - something spoken that is true (note kanji for to speak), 明るい (あかるい) - akarui - bright/clear, 明く (あく) - aku - to open, 明々(めいめい) - meimei - crystal clear
Duo will not allow a reply on your reply, how odd. Yes, I meant you Ana... Who else? lol.
The comment number-of-responses icon was not explained and i had no idea there were even comments. I thought the silly bird was as deep as it got.
Anyway, I'm learning considerably more thanks to efforts like yours. What alternate sources do you use, seeing as your language exploration is wide and levels elevated. Except for English, your investment seems poor there. Needs work.
Yes, there is a limit to how many times you can reply to people's comments but I'm not sure what the limit is.
Anyway, I don't really have any alternate sources - for Japanese I read books and talk to friends (I have a Japanese keyboard on my cell phone, tablet and PC) on messenger or via video chat or in person, I deciphered a letter for a friend from her Japanese Uncle the other day (she is half and can read some Japanese but can't really speak it), I have a few apps on my phone but I don't really use them, and I studied Japanese at high school, did a couple of uni courses a few years back and lived in Japan for just over a year. As for the other languages I'm studying - I love languages so that's why I study so many. I studied Spanish in my last year at high school and that was what actually got me interested in Duo - I thought I would revise my Spanish.
As for English, it is my first language - hence why I don't study it on Duo. I've just looked at my profile and realised the source of your confusion - I think I clicked on English for a joke once or I might have accidentally set my account to Greek and was trying to get it back to English. Anyway, the English course is still appearing on my profile even though I removed it ages ago - it doesn't appear on my drop down menu of languages or in my list for adding or removing languages. Yet another charming Duo glitch : )
入る ・入ります will almost always be read as "hairu/hairimasu" for the intransitive verb "to enter/go in". "Iru" is an antiquated reading you will mainly only see in compound nouns and set expressions now.
Then there's 入れる ireru - "to put in", the transitive form of the verb.