This is the first time I've seen this word and I don't know what it means.
I've never seen this word before and I don't understand what it means.
1) This is the first time I've seen this word and I don't know what it means.
「この単語を見たのは初めてなので、意味がわかりません。」
「今回初めて」は this is the first time
2) I've never seen this word before and I don't understand what it means.
「この単語を今まで見たことがないので意味がわかりません。」
「今までに〜を見たことがない」は I've never seen ~
「意味がわからない」は
I don't know what it means
I don't understand what it means
ご参考になれば幸いです!
This is the first time I have seen this word so I don't understand it.
長い文なので、区切って訳・解説します。
This is the first time →これが初めて
I have seen this word → この単語を見たのは
so → なので
I don't understand it → 意味が分からない、理解できない。このitはthis word「この単語」を指します。