了解

知道 is a light switch — you either know the fact or you don't. 了解 is a dimmer switch, and it only turns up after real time and contact. You can 知道 that Beijing has old teahouses, but you only 了解 old Beijing culture once you've sat in one and drunk the tea. So the writer picked 了解 on purpose: he means lived, firsthand familiarity, not a fun fact. Swapping in 知道 would flatten the whole sentence into tourist trivia.

“前天有个朋友告诉我，要想了解老北京的文化，应该去老茶馆喝一次“大碗儿茶”。” (The day before yesterday, a friend told me that if you want to understand old Beijing culture, you should go to a traditional teahouse and try 'big bowl tea' at least once.)

认识 is about the connection existing — you got introduced, you added them on WeChat, you nod when you pass. 了解 is about depth: their moods, their habits, what they're like at 2am. You can 认识 someone for ten years and still not 了解 them, which is basically the plot of half of Chinese TV. So 我了解他不是 'I know him', it's 'I've got his number'.

Bare 了解 sounds like a serious commitment, almost like a research project. Adding 一下 softens it into 'let me have a quick look', which is why shop assistants, doctors and HR all use it constantly. When a salesperson says 您了解一下, she isn't demanding you master the product, she's inviting you to glance. And 我先了解一下 is the polite way to tell your boss 'let me check and get back to you' without promising anything.

For language ability, Chinese uses 懂 or 会说, because those describe a skill living inside you. 了解 wants a topic as its object — a culture, an industry, a situation. If you tell a colleague 我了解中文, they'll picture you reading grammar papers, not chatting with the delivery guy. Keep 了解 for things you've gotten to know, and 懂 or 会 for things you can do.