"I like milk."
Would it be wrong if I would translate it as "Lubię mleko."? Because in some lessons someone explained that it is not needed to write Ja when it is shown in lubiĘ.
If you use an English keyboard, you can use Polish one instead - it has everything that you need for English and everything that you need for Polish.
Well... I'm afraid that such a general question is rather difficult to answer.
Here you have:
"lubię" (the pronoun 'ja' is omitted because it's redundant - the 1st person singular form "lubię" makes it obvious what the subject is)
and "mleko" (it looks the same as Nominative, as it's neuter - but it's actually Accusative.
Accusative is probably the most common case, and it's needed by a lot of verbs, including "lubić").
Probably cases. But "mleka" is Genitive, and I don't think that you already reached the moment when Genitive was introduced.
Some lessons I only know the starting word because it is capitalized for the start of the sentence