Gloss on words in the passage 'Once or twice a minute all summer' -- Harry Potter Japanese translation.

Ō 'oh' 'oh Percy'
パーシー Pāshii 'Percy'
kimi 'you' (familiar) 'have you become a Prefect?'
監督生に kantokusei ni 'prefect' + ni になる ni naru means 'to become (something)'
なった natta 'became' (past tense of なる naru)
のかい no kai sentence-final particle (question). This asks for confirmation of something that the speaker suspects.
'Oh Percy, have you become a Prefect?'
futago no 'twin' + no 'one of the twins' (subject) one of the twins said deliberately as though surprised
一人が hitori ga 'one' + subject
わざと waza-to 'deliberately'
驚いた odoroita 'be surprised (past tense of 驚く odoroku 'to be surprised') 'as though surprised, in a surprised way'
ように yō ni 'as though' (よう is technically a noun meaning 'manner, appearance' and is modified by 驚いた odoroita. ni makes it adverbial: 'in a manner of being surprised')
言った itta 'said' (past tense of 言う 'to say')
One of the twins deliberately said in a surprised tone
そう 'so, in that way' 'you should tell us so'
言ってくれれば itte kurereba 'if tell us' (conditional form of 言ってくれる itte kureru 'to tell us'. -te kureru indicates an action done (1) for the benefit of the speaker by the person addressed or a third person, or (2) for the benefit of the person addressed by a third person. ばいいのに -ba ii no ni means 'even though it would be good if you...' In other words, 'why don't you?'
いい ii 'good'
のに no ni 'though'
知らなかった shiranakatta 'didn't know' (past tense of 知らない shiranai 'to not know', from 知る shiru 'to know, to come to know') '(we) didn't know, did we?'
じゃない ja nai 'Is not'. Abbreviated spoken form of ではない de wa nai, the negative of である de aru 'to be'. Together じゃないか ja nai ka is a fixed expression making a strong appeal to the other party to recognise the correctnesss of one's assertion.
ka Sentence-ending particle (question)
'You should have told us! We didn't know, did we?'
まて mate 'wait!' (imperative of 待つ matsu 'to wait')   'Wait!'
yo emphatic particle (emphasising 'wait!')  
そう 'so, in that way' 'If (one) says that' = standard phrase for 'now that you/I mention it'. 'Now that you/I mention it'
いえば ieba 'if say' (conditional form of 言う 'to say')
なんか nanka 'something, somehow' Used as a vague expression of trying to remember something '(I kind of remember) he was talking about that sort of thing one time in the past'
以前に izen ni 'in the past, before'
一回 ikkai 'one time'
そんな sonna 'such, that sort of ' 'that sort of thing' (object of 言ってた itte-ta)
ことを koto o 'thing + object particle'
言ってた itte-ta Abbreviated spoken form of 言っていた itte ita 'was saying'
na sentence-ending particle (invites agreement)
to quotation particle (quotes the foregoing) (quote)
もう 'more, another' もう一人の mō hitori no means 'another one', 'the other one', where 'one' refers to a person. another of the twins (said)
一人の hitori no 'one person' + no
双子 futago 'twin'  
'Wait, now that you mention it, (I kind of remember) he was talking about something like that once in the past', (said) the other of the twins.
Note: Omitted after 双子 futago 'twin' but understood is the subject particle ga and the verb 言った itta 'said'.
二回 nikai 'two times' 'maybe twice'
ka sentence-ending particle (question) かな ka na is used as a kind of query to oneself, used for tentative statements -- 'maybe?' or 'I think'
na sentence-ending particle (invites agreement)
'Maybe twice'
一分間に ippun-kan ni 'in one minute' 'Maybe once or twice a minute'
一、二回 ichi, nikai 'one or two times'
ka sentence-ending particle (question) かな ka na is used as a kind of query to oneself, used for tentative statements -- 'maybe?' or 'I think'
na sentence-ending particle (invites agreement)
'Maybe once or twice a minute'
夏中 natsu-jū 'all summer' 'was saying it all summer'
言ってた itte-ta Abbreviated spoken form of 言っていた itte ita 'was saying'
ような yō na 'type of' Technically, よう is a noun meaning 'appearance', 'manner'. na then connects to a following 気がする ki ga suru 'to have a feeling, impression (that)', which is omitted but understood. '(have) a kind of (feeling) that'
'(I) have a kind of (feeling) that he was saying it all summer'
だまれ damare 'be quiet' (imperative of だまる damaru 'to fall silent') 'Shutup!'
to quotation particle (quotes the preceding).
監督生パーシーが kantokusei Pāshii ga Prefect Percy + subject particle Percy Prefect said.
言った itta 'said' (past tense of 言う 'to say')
'Shutup!' said Percy Prefect.

It is difficult to gloss Japanese sentences on a word-for-word basis because Japanese has a completely different word order to English. The logic, however, is not hard to master. The knack is remembering that (1) the verb comes at the end, and (2) within a sentence, where a noun is preceded by a verb, the verb modifies the noun.

Thus, パーシーがそれを言った Pāshii ga sore o itta 'Percy that said' is the normal word order in a sentence meaning 'Percy said that'. The particle ga indicates that 'Percy' is the subject of the verb. The particle o indicates that それ sore is the object of the verb.

When the order changes to それを言ったパーシー sore o itta Pāshii 'that said Percy', the verb 言った itta 'said' is regarded as modifying 'Percy'. In other words, the meaning becomes 'Percy who said that'.

This logic is carried through to all kinds of constructions. For instance, よう 'appearance' occurs twice in the passage above and functions as a noun that is modified by that which comes before. The eventual meaning is something like saying 'it seems...' in English.

Another problem area is the use of sentence-ending particles. The most common of these are か ka (indicates a question), よ yo (indicates an assertion), ね ne (indicates an appeal to the listener, like 'isn't it?' in English), and な na (similar to ね ne but less polite, often used in 'speaking to oneself'). There are quite a few other such endings, including compound endings where two particles are used together, and it can be difficult to indicate the exact nuance in English. One problem is that while English has similar sentence endings (e.g., 'huh', 'eh', etc.), they are regarded as quite colloquial and not suitable in written contexts.