First of all, originally there is a difference between the prononciation of both letters. The letter ט has a deeper, more guttural sound than the letter ת, and so the two words should be read differently and looked at and treated as two completely different words. Different letters, even if the prononciation is similar, always have an historical background behind them. Since it doesn't exist in English let's just write ט as a capital T and ת as a lowercase t for the moment. Now, even though the difference is not as noticeable in Modern Hebrew, while the name Tal means "dew" the word that you wrote is read as "tel" and means something along the lines of "man-made mound" (for example the name of the city "tel-aviv" literally means "spring mound"). Funnily enough "תל" with the sound "tal" does not appear on its own, but when it is doubled it creates the word taltal which means "curl" (like hair curls). I hope it was helpful and not too confusing!
Very good explanation, I should add that Arabic (another Semitic language related to Hebrew) retains the phonetic distinction between the two t's: ت and ط. I also read somewhere that Jews originally from Arabic-speaking countries also tend to pronounce ת and ט differently because of this.
Literally they say "you will enjoy".
The hebrew verb for to enjoy has another ethymology as the noun fun
Verb "to enjoy" לֵיהָנוֹת : Root: ה - נ - ה
Future tense, 2nd person, feminine, singular: you f. sg. will enjoy: תֵּהָנִי ~ תיהני tehani
Noun "fun" כֵּיף • (keif) : from Arabic kayf كيف
Not sure what you're asking. תהני is 2nd person - the ת at the beginning is a clear sign (for verbs in future tense, often used - like here - as imperative). The conjugation of "I" in future starts with א (for this verb, אהנה /ehane/). But you have to know the verb to know that the ת is a a conjugation prefix, so for now you might want to just accept it...
Sorry, but Yarden is a native speaker and he most certainly is correct. That is future tense used as imperative. It's a common phrase, that's why it's taught in the phrases skill. We are also taught מדבר even though pi'el skill is much further down the tree, and להתראות even though infinitives skill is also a long way down. They are all useful phrases, no matter what the grammar behind them is.
Yarden wasn't understanding what they were asking about. I was merely pointing out that for a learner who knows nothing of tenses at this stage, all they know is that the י suffix is associated with the first person. Yarden gave a great explanation, but I understand why the learner was confused originally and wanted to make it known why they were confused.
Are you sure you saw "תהנו"?
It can be equally well תהני and תהנה; the first would be used if you wish fun to a woman named Tal, the second to a man named Tal.
The form תהנו is addressing multiple people, so can't work like this. I can imagine one context for תהנו, טל, a context which is esoteric these days: if you write a letter addressed to multiple people, and you conclude the letter with wishing the recipients fun, and then sign with your own name, which happens to be Tal...