I'd like to know how to analyze "是其于主也至忠矣" word for word.
I would appreciate your help!
The verse is from 戰國策 卷二十八:
Though the OP asked one particular verse, and we must comprehend the related text (from page 80-82), in order to interpret it correctly.
The context is:
Someone suggested (或謂) to mr 韓公仲 (公仲), said (曰) "there's now (今有) a strategy (一舉) that is loyal to the lord (忠於主), convenience to the nation (便於國), benefit to yourself (利於身), hope (願) your honourable (公) execute it (行之)
So "公行之計．是其於主也．至忠矣", I would interpret it as:
[If] your honourable (公) execute (行) this (之-->此) strategy (計), such execution is (是其), for the lord (於主), absolutely loyal (至忠).
是其, reverse it to 其是, so
If] your honourable (公) execute (行) this (之-->此) strategy (計), such execution (其) is, (是) for the lord (於主), absolutely loyal (至忠).
也 and 矣 is, particle.
Have fun :)
公 - Sir (honorific)
行 - implement; carry out (v)
之 - possessive form: similar to ['s ] in English
计 - plan (direct object)
是 - affirm (v)
其 - it (pronoun)
于 - to
主 - master (indirect object)
也 - (= 也算) can be considered
至 - utmost
忠 - loyal
矣 - final particle (for emphasis)
The plan he came up with, suggest it to his master can be considered a showing of his utmost loyalty,
To analyse the quote, I would dive into the original text and read the article.
The part you quoted is from 或谓公仲 in 战国策，where 或 means someone. The article is documenting someone trying to lobby the lord of 韩国 to be the first one to show loyalty to 秦国。By that time, 秦国 is getting strong and starting to show the ambition of unifying the whole China. That someone is trying to persuade the lord of 韩国 that given 韩国 is relatively weak, showing loyalty to 秦国 is a wise choice and the earlier, the better.
The quote you have, is a small summary of the advantages, it means
I will write the part of analyzing word by word mostly in Chinese as it is more accurate doing it this way