Doing X is …: a verb phrase as the subject, no 'to' or '-ing' (Học tiếng Việt khó)

An activity can be the subject of a sentence just as it is: Học tiếng Việt khó. Learning Vietnamese is hard. English needs "learning" or "to learn"; Vietnamese simply starts with the verb, which never changes, just as when two verbs sit side by side.

A describing word follows the activity directly, with no là, as after any other subject: Dậy sớm mệt lắm. Getting up early is so tiring. Chạy xe ở Sài Gòn hơi sợ. Riding a motorbike in Saigon is a bit scary (in Hà Nội, đi xe máy).

Thì after the activity makes it the topic, often to set up a "but": Học tiếng Việt thì khó, nhưng vui. Learning Vietnamese is hard, but fun.

Là comes in when a noun follows, saying what the activity is: Đi du lịch là sở thích của em. Travelling is my hobby. Đọc sách là cách học tốt nhất. Reading is the best way to learn.

In formal writing, việc often goes in front to make the activity a clear thing: Việc học tiếng Việt cần nhiều thời gian. Learning Vietnamese takes a lot of time.