No -- there is a difference in the Duolingo font: the small L has a small curl at the bottom while the capital i is completely straight. Have a look: l I l I l I, l I l I l I, l I l I l I.
I'll admit that it's very subtle.
If you're using Google Chrome, I'm told that there's a font changer in the Chrome store that will let you override the font for a website and choose something such as Verdana that has a more pronounced difference between capital i and small L.
It can also help to know that Klingon has a very simple syllable structure with essentially no consonant clusters (except when one consonant at the end of one syllable meets one consonant at the beginning of the next one), so all small Ls must be next to a vowel on at least one side (or be part of the spelling of the Klingon letter tlh, which has a small h in its spelling), and all capital i's must be next to a consonant on at least one side.
So bISup can only have a capital i as its second letter, because you couldn't have three consonants b-L-s in a row like that in Klingon.
The "Klingon" forum would be a good place to start: https://forum.duolingo.com/topic/953
I'm not sure whether the mobile apps give access to the general forum, or only to individual sentence discussions.
If not, yet another strike for mobile apps, because important information might be posted in the forums, and non-sentence-specific discussions should go there as well, but if a mobile app user isn't even aware of their existence, then... sigh
Is there any distinction between past, present, and future tenses?
No. Klingon does not mark tense on its verbs.
What it does mark is aspect -- whether an action is ongoing or has an end.
But vISoppu' (for example) could mean "I had eaten it" or "I have eaten it" or even "I will have eaten it" -- the aspect is perfective (an end is implied) but the tense is not specified (this completed eating could have taken place in the past or it might still take place in the future).
Similarly, mara vIlegh could mean "I saw Mara" (in the past), "I see Mara" (in the present), or "I will see Mara" (in the future).
And paq vIlaDlaH could mean "I was reading the book" (in the past), "I am reading the book" (right now), or "I will be reading the book" (in the future) -- the aspect is ongoing, but the tense is unspecified.
As far as i know there is no grammatical tenses n Klingon. Its provided by the context. E.g. you name the exact time.
I'm also annoyed by all this tense expectations for the translation. Especially as we are speaking about the first lessins. I've done basic lessons for other languages and they are usually solely in present tense, to not complicate it.
You are correct, there is no grammatical tense in Klingon. Duolingo sentences generally lack context and we don't introduce time stamps until later, thus it is true that Klingon sentences without context can be translated in the present, past, or future tenses in English. You are not required to translate Klingon sentences in a particular tense unless some context clues have been given. Throughout the course, in general, you may translate a Klingon sentence with any of the three basic English tenses. No particular tense is required.