Software products are available that can help with this problem. 請問that的用法是什麼? 形容詞子句? 名詞子句? 或是其他呢? 感謝回答?

樓上的匿名回答，相信沒人會看，我懷疑他自己也看不懂，只好再代勞一下
That常用於一段話後面用來補充說明前句 – 也就是說啟始另一個子句 – 知道這就好 – 至於這叫什子句 – 我不知 – 您要是問大部分老外 – 他們更是不知 – 只有補習班老師及硬梆梆的文法書知
好了，既是子句應有主詞及動詞，這時候主詞可能是 –
That後接的一個字或一段話(片語)，如that 後面只有動詞沒有主詞
，那that本身有可能便是虛擬主詞(我沒文法書我幾乎沒看過文法書-故有可能自創名詞)，意即that之前的敘述句之主詞也是子句的主詞，好了翻譯如下
Software products are available : 各種軟體隨手可得 – 到處買得到
that : 這些軟體 – 如前述that後是動詞沒主詞 – 那that之前所敘述句之主詞便也是子句的主詞， 故在此句 that = 這些各種軟體
can help with this problem : 能夠幫忙解決這些問題

Software products that can help with this problem are available.
要寫成上面這樣，句子才正確。修飾名詞的子句（形容詞子句）應該緊跟在那個名詞（先行詞）的後面，不可落在主詞補語available的後面。

>>請問that的用法是什麼? 形容詞子句? 名詞子句? 或是其他呢?
這都不是重點，重點在於為什麼句子可以這麼說? 又是如何演變。

Software products are available that can help with this problem.
= Software products that can help with this problem are available.

中文文法書說關係代名詞緊接先行詞之後，這是一個迷思。
A relative clause does not in general begin with a relative pronoun, but rather with a relative expression within which a relative pronoun occurs (as for instance this very relative clause, which begins with "within which").
http://english.stackexchange.com/questions/325536/relative-pronouns-are-always-added-right-after-antecedent

Restrictive relative clauses can sometimes be extraposed to the end of the verb phrase, as in "A man appeared who had a sinister red beard."
限定用法關係子句可置於動詞片語之後，are available 在整個句中是一不可分割的動詞片語。

原句的使用在問句中就顯的自然
What are available that can help with problem?
What treatments are available that lessen the condition of PWS
What tours are available that can be booked at the hotel?