太 (tài) goes before an adjective, and 了 (le) usually comes at the end of the sentence. Together they say that something is too much, as in "too expensive", or that you feel strongly about it, as in "so good". The context tells you which.
太 + adjective + 了. The adjective sits between 太 and 了, and the 了 here does not mean the action is finished; it just adds feeling to the sentence.
太好了 on its own is one of the most useful phrases in Chinese: "Great!" or "That's wonderful!"
When something is a problem, 太……了 means "too". It is often followed by a second part that says what you will do about it: ask for something else, say no, or put it off.
With positive words, 太……了 is not a complaint at all. It means "so" or "really" and shows that you are pleased or impressed. 太谢谢你了 is a warm way to say "Thank you so much".
Put 不 in front of 太 to say "not very" or "not really". 不太 sounds softer and more polite than a plain 不, so it is a good way to say you don't like something. Normally there is no 了 at the end. Note the tone: 不 becomes bú before the fourth tone of 太.
To ask whether something is too much, you can use 是不是 before 太: 是不是太贵了?
Learners often use 太 when they just mean "very". 太 is not a neutral word: it says something is too much, or that you feel strongly about it. To simply describe something, use 很. 这个苹果很大 just says the apple is big; 这个苹果太大了 says it is too big, or that you are surprised how big it is.
The other mistake is leaving out 了. When the sentence ends on the adjective, keep the 了: say 这个太贵了, not 这个太贵.
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