Learn Hardcore Japanese: atarasii ie no kagi wo motte imasu.

新しい is an i-adjective, so it directly modifies the noun 家 without needing の. You only need の when linking two nouns (e.g. 息子の本 “son’s book”).

What does the particle を indicate in 鍵を?

を marks 鍵 as the direct object of the verb 持っています. It tells us what is being “had” or “held.”

What does the form 持って　います mean? Why not just 持ちます?

持っています is the -te form of 持つ + the auxiliary いる, creating the progressive/state aspect, so it means “to be in possession of” or “to have.”
持ちます is the simple non-continuous form (“I will hold” or “I hold”), but Japanese normally uses 持っている for ongoing possession.

Why do we use 持っている instead of 持つ when talking about possession?

In Japanese, persistent or current possession is expressed with the progressive/state form 持っている. Using just 持つ can sound like a one-time action (“to pick up” or “to carry temporarily”).

Why is there no subject like 私は at the start?

Japanese often omits the subject when it’s clear from context. Here, it’s understood you’re talking about yourself, so 私は is optional.

Why is the verb at the very end of the sentence?

Japanese follows a subject–object–verb (SOV) word order, so the verb 持っています naturally comes last.

Could we replace 持っています with あります?

No. あります is the existential verb for inanimate things (“there is/are”), not the progressive “have.” To express “I have X,” Japanese uses 持っている.

How would you ask “Do you have the key to the new house?” in Japanese?

Add the question particle か at the end:
新しい家の鍵を持っていますか？

How do you make this sentence negative?

Turn いる into its negative form いません (polite) or いない (plain):
新しい家の鍵を持っていません。
or (plain) …持っていない。

How do you specify that you have two keys?

Use the counter 本 for long, slender objects like keys:
新しい家の鍵を二本持っています。