Need help with this text in Latin

Hi,
I wonder if anybody out there can help me out. This is a tricky text because it was written a couple of centuries ago by somebody who did not have a clear grasp of Latin - but surely better than mine . Also the author is trying to explain in Latin how to address certain grammatical issues in another language. So this is the text: "Modus infinitus verborum erit XY. Haec litera mutatur xxxx pro indicativi tempore presenti xxxx singulari numero xxxxx numero plurali, nisi infinitus erit in xxxx." This is what I came up with: "the infinitive mode of verbs is XY. This letter will be changing to xxxx for the present indicative tense in the singular number xxxxx, [to] xxxx [in] the plural number, unless the infinitive ends in xxxxxx. Thanks in advance! Cecilia