Only the context in general will give you the different meaning.
eg:- Dych chi'n yfed te? Ydw dw i'n yfed te. = Do you drink tea? Yes I drink tea.
Dych chi'n yfed te nawr? Ydw dw i'n yfed te nawr. = Are you drinking tea now? Yes I'm drinking tea now.
There is a form of the verb which is nowadays only the future tense but in the past would have been used for the habitual action.
Yfwch chi de? (Will you drink tea? ('Do you drink tea' also previously)
Yes, there are similarities with 'Je ai" contracting to 'J'ai' in French.
The 'yn' connects the verb to be to the following verb or adjective to make the sentence.
eg Dw i yn bwyta cacen = I am eating a cake where the yn connects the I am with eating
This is contracted in normal speech to Dw i'n bwyta cacen although for emphasis the full form is used.
The reason for the contraction is that there are two vowels (y is a vowel in Welsh) next to each other in this sentence and it's easier to drop one when you say the sentence.
So, 'yn' always comes before verbs? Is that why we never find it before 'eisiau', because it is a noun? So, 'Dw i eisiau siocled.' is literally "I am [in] want [of] chocolate."? What does 'yn' actually mean in Welsh, if it has a literal meaning at all? Sori for the multiple questions