It is important to master certain abilities and elements when learning English. Understanding English grammar is among those abilities that learners find the most challenging. Focusing on the infinitival clause, this paper analyses the syntactic structure of to- infinitive phrases in English that function as adjunct and subject, focusing on the D-structure that may characterize them. The finding shows that: 1) to- construction happens in infinitival clauses, which in its D-structure, is within the Tense Phrase (TP) construction, 2) infinitival clauses that function as an adjunct or subject are generally without any specifier (DP) of the TP since the to- construction usually does not allow a subject within the infinitival clause, 3) In some cases, the to- construction allows the occurrence of Spec/T for sentences that are constructed to have an NP precedes the to- + infinitive form and is a part of the infinitival clauses, and 4) to- construction can be arranged into adjuncts that occur at the beginning of sentences.