How can recovered 5-letters plain text help me to recover reused OTP key
Assume that $P_1$ contains " the ". In that case you can get the key stream by XOR'ing " the " with $C_1$, lets call this key stream $K^1$. If this key stream is correct then $P_3^1$ should make sense, where $P_3^1 = K^1 \oplus C_3$. If $P_3^1$ doesn't make sense then you can create $K^2$ and $P_3^2$ from $C_2$ in using an identical calculation and check that.

Of course, as you've got 10 ciphertext you might as well check against $C_3$ to $C_{10}$. They all should make sense if you've got the right key stream at the location you are testing. Beware though that some professors find it funny to introduce single "mistakes" in the plaintext, forcing you to perform statistics (or "by eye" verification) instead.