Q:

Question about "nari"

七三年にはテレビ番組になり、今も毎週続いています。

I understand the sentence to be:

In '73 it became a television show, and today it continues every week.

My question is about the form of the verb なる in the above sentence – why is it "なり" here? When is the stem form of the verb used in this way?

A:

「七三年{ななじゅうさんねん}にはテレビ番組{ばんぐみ}になり、今{いま}も毎週続{まいしゅうつづ}いています。」

As you know, both verbs and adjectives conjugate in Japanese.  「なり」 is the 連用形{れんようけい} ("continuative form") of the verb 「なる」.
This sentence talks about two events -- 1) "It became a TV program." + 2) "It has continued to today."
「なる」 is the terminal form; therefore, it cannot be used mid-sentence to connect itself to another verb phrase.  Instead, you must use the continuative form 「なり」. 
Read here and see how 「だ」 becomes 「で」 mid-sentence:
How to parse 中国人で日本語が話せる方は、お電話ください。

