我們有時會在 be、claim、expect、hate、hope、like、love、prefer、pretend 和 seem 等動詞後面使用完成式不定詞 (perfect infinitive，即 to + have + P.P.) 來表示該完成式動作比這些動詞還要早發生，但完成式不定詞的動作可能是過去的事實，也可能是過去的非事實 (與過去事實相反)。
完成式不定詞的動作係過去的事實。例如：
- Peter claims to have met the president, but most of us don’t believe him. (彼得聲稱見過總統，但我們大多不信) － 至於彼得是否真的見過總統並不重要，也不影響它在英文中被視為「過去的事實」這一事實。
= Peter claims (that) he has met the president, but most of us don’t believe him. - I seem to have lost my way. (我好像迷路了)
= It seems to me that I have lost my way. - He seems to have gone out. (他似乎已經出門了)
- John hated to have quarreled with his wife. (跟太太吵了一架，約翰感到遺憾)
完成式不定詞的動作係與過去的事實相反。這種過去的非事實主要用在下列句型：
Be to + have + P.P. － 這是表示過去計畫好或安排好的事情沒有實現或發生，所以這裡的 BE 動詞只能用 was/were。這相當於在口語上用得比較多的 was/were supposed to。例如：
- Jack was to have gone on holiday with Miss Chen, but she couldn’t get time off work. (傑克原本要和陳小姐去度假，但她無法請假)
= Jack was supposed to go on holiday with Miss Chen, but she couldn’t get time off work. - They were to have met me in the library, but they didn’t get there. (他們原本要跟我在圖書館見面，但他們沒有到)
= They were supposed to meet me in the library, but they didn’t get there.
Would like/love/prefer to + have + P.P. 或 wanted + to + have + P.P. (注意：這裡 want 必須用過去式)。例如：
- My brother would like to have studied abroad. (我弟弟本來想出國留學 － 但事實上並沒有出國)
= My brother wanted to have studied abroad.
= My brother would have liked to study abroad. － 我們亦可用 would like 的過去式 would have liked + 現在式不定詞 (to + 原形動詞) 來表示過去的非事實。 - I would like to have gone to Pescadores for the weekend. (我本來想去澎湖度週末 － 但事實上並沒有去)
= I wanted to have gone to Pescadores for the weekend.
= I would have liked to go to Pescadores for the weekend. - I would prefer to have worked for a small company than a big multinational conglomerate. (我寧願在小公司工作，而不是在大型跨國企業集團 － 但事實上他並非任職於小公司)
Pretend to + have + P.P. 也是完成式不定詞的動作與過去事實相反的句型。例如：
- She pretended to have just broken up with her boyfriend. (她假裝剛與男朋友分手)
= She pretended that she had just broken up with her boyfriend. － 這句 pretended 為過去式，所以比過去 (簡單) 式更早的動作要使用過去完成式。
完成式不定詞除了可以表示該完成式動作比主動詞還要早發生之外，亦可表示該完成式動作比主動詞還要晚發生 (指將在未來某個時間點完成的事情)。例如：
- I hope to have got a doctor’s degree in physics from MIT by the end of July. (我希望在七月底之前取得麻省理工學院物理博士學位)
- Salary growth has been weak for 2017 and is not expected to have changed significantly for 2018. (薪資成長在 2017 年一直處於疲軟狀態，預料在 2018 年也不會有重大變化)
現在式不定詞 (to + 原形動詞) 可以用作名詞來當句子的主詞；同樣地，完成式不定詞 (to + have + P.P.) 亦可用作名詞來當句子的主詞，而且可藉由主動詞的時態變化，讓完成式不定詞的動作可以是過去的事實，也可以是過去的非事實。例如：
- To have got a doctor’s degree in physics from MIT was a great achievement. (取得麻省理工學院物理博士學位是一項重大成就) － 此人已取得該學位。
- To have got a doctor’s degree in physics from MIT would have been a great achievement，but that was just a dream. (取得麻省理工學院物理博士學位會是一項重大成就，但那只是個夢想) － 此人並未取得該學位。