The sentence you provided is mostly correct, but it can be made a bit clearer and more natural. Here is your sentence:
The koala has two big fingers so it can securely grab onto the branches.
Here's a slightly improved version:
The koala has two big fingers so it can securely grab onto branches.
Explanation:
- "The koala" is correct if you are talking about koalas in general. You can also say "Koalas have two big fingers..." if you're talking about all koalas.
- "Securely grab onto the branches" is grammatically correct, but we usually say "grab onto branches" without "the" when speaking generally.
- So, removing "the" before "branches" makes the sentence smoother.
Therefore, your sentence is good, just a small change to improve it.