1. There is no difference, but "these days" often implies a longer time span: - These days, apartments in Seoul are expensive. - Kids these days are disrespectful. 2. Basically no difference, bit "in it" can also mean "mixed into" the other ingredients: - My coffee has cream in it. - My bread has raisins in it. "Inside" can only mean that the beans and custard are in the center of the pastry, and not mixed into the batter. 3. Very little difference. I guess = I think, I suppose, I presume. Probably = it is likely.

1. It's been bitterly cold here in Korea. (If you're speaking it (versus writing), you don't really need the word 'lately' because it's already indicated that it has been cold with the word 'been'.) 2. It has either sweet red beans or custard cream inside it. (The 'or' indicates that the pastry has one or the other.) 3. Both sentences are correct...it just depends on the context of what you are talking about. :)

1)"These days/Nowadays/Presently/Lately it's bitterly cold here in Korea." = "It's been bitterly cold here in Korea lately/these days etc " 2)"It has only (just) some sweet red beans or custard cream in it." = "It has only (just) some sweet red beans or custard cream inside." 3)"I guess I ( have) made a lot of mistakes" = "I probably made a lot of mistakes."