there are 2 rules to make the adjective/verb into a formal declarative sentence.

• [A/V stem + ㅂ니다] if the stem has no batchim (ending consonant) or its batchim is ㄹ.

• [A/V stem + 습니다] if the stem does have a batchim.

here the verb is 넘어지다 and its stem 넘어지. because there is no batchim, you then apply the first rule which would make 넘어집니다. hope that helps..

If the stem ends in a vowel, you add ~ㅂ니다. If it ends in a consonant, then you add ~습니다.

Here the verb is 넘어지(다), so you have 넘어지 + ㅂ니다 = 넘어집니다

When you remove the -다 to form your verb tense, if the verb ends with a vowel sound (이, 아, 에, 유, etc.), you combine it with -ㅂ니다. Ex. 사랑하다 (to love) = 사랑합니다

When you remove the -다 to form your verb tense, but the verb ends with a 받침 (consonant sound - ㄱ, ㅈ, ㄹ, ㅁ, etc.), you add 습니다 to the end. Ex. 먹다 (to eat) = 먹습니다

Hope this helps! 파이팅!

Because the infinitive verb is 넘어지다. Without 다 is 넘어지 it ends with vowel so you need to ad ㅂ니다. 넘어집니다...

In a sentence this small it doesn't matter what you use. It's much more obvious when you have more details. If you say something like "I think my dad is funny" instead of having to translate "I think" you can just say 저는. That's like a shortcut to say "according to me."

저는 아빠가 웃겨요.

Let's try another one. "Did mom eat my strawberries?" 엄마는 나의 딸기를 먹었나?" Mom is the topic of the sentence so it gets 는, strawberries are the direct object so they get 를. Let's change it to add 가/이. "For dinner, mom will eat my strawberries." - 저녁은 엄마가 나의 딸기를 먹을 거야. Here dinner is the main topic (는/은), mom is the subject (이/가) that acts on the object (를/을).