but i learned that the ń is pronounced like " ni
someone mislead you the ń sound is just a consonant like ng in champange -
ni compound is a consonant plus i vowel -
nikt (nobody) - the i softens the n and makes it sound ń while the i wovel is still heard - you can phonetically write down the compound as ńi
thus słoń and słoni sound differently (słoń and słońi) - słoń is elephant
there are however further compounds with ni: nie, nia, niu etc - in this case i letter only marks the softening of the n letter into ń and is not heard as a vowel i - so in effect you phonetically write these compounds as: ńe, ńa, ńu etc
ę is pronounced at the end of words as plain e - so in the case of tęsknię you phonetically write it down as tęsknie (or tęskńe taking into account what I have written above)
ę in the middle of the word can be approximated by en but it's not proper speech