very hard to remember especially while having a conversation with native speaker...

btw,malay language is i think the most easiest languages to learn

believe me...

The most difficult thing in Portuguese for me is the pronunciation, Portuguese has a lot of vowels that I cannot pronounce well, and the structure of the phonetic is different to my native language and as Portuguese has a lot of similaties with my native language, I usually made a lot of mistake.

In my native language, Spanish, the most difficult things are the grammar, because Spanish has a lot of conjugations of verbs, regionals words, becuase a lot of words common in an area are not in common in others areas, for example the word ´bus´ is ´autobús´(Spain) , guagua (Dominican Rep, Cuba and Puerto Rico), camión (Mexico), ónibus (Peru), colectivo (Argentina) , and the pronunciations, becuase there are sounds difficult for the everyone who is learning Spanish like R, r, rr and the pronunciation can change a lot of in an informal conversation. 

I'm trying to learn Hindi now and the difficult things are the writing system that makes it very slow reading, and the sentence structure. Sometimes I understand every word in a sentence but I still don't know what it means. 

In my native language, Finnish, the most difficult thing is probably the word inflection. There are a million forms of nouns, adjectives and verbs.  

In French I'd say the pronunciation.

In my native language ,Hungarian,we have a lot of suffixes for example there is the world time in Hungarian it is "idő" but if you want to say in time then it becomes "időben" and so on.And the pronunciation is quite hard for a a native English speaker in my opinion.

For me the most difficult things about German it's been the cases and word order... Sometimes they drive me nuts!

About Portuguese, I think our Grammar is really complex... teachers tend to teach some structures and pronouns we don't use in a normal conversation... also the pronunciation must be really confusing...