請問這句子的to 是什麼詞

substitute products constrain the ability of firms in the dustry to raise their prices substantially

可改寫為 Substitute products constrain firms in the industry the (their) ability to raise their prices substantially.

to raise their price substantially = an adjective clause
修飾名詞 the ability
所以不能只用動詞 raise, 必須加 to 成為 an adjective clause
與可否用兩個動詞無關

請問以上to 是否介系詞 ? - Yes

如果是介系詞,為什麼後面不是動名詞? – because the preposition “to” is used as part of to-infinitive clause

如果是不定詞,為什麼要用不定詞?因為這是子句,應該可直接用動詞 – This is a to-infinitive clause so we cannot just use the verb.

Let’s see how the sentence will become if we just use verb :
Substitute products constrain the ability of firms in the industry raise their prices substantially. – You can see this is incorrect grammar.

To make it grammatically correct we have to change the verb “raise” to a gerund, with a suitable preposition in front of it :
Substitute……ability….. in raising their prices…..