"Functional tenses” are aspects and moods.
Huddleston and Pullam, in The Cambridge Grammar of the English Language, describe two systems of tense, one system of aspect, and one system of mood in English.
The primary system of tenses are present and preterite (past), both inflected. The secondary tense system is the perfect: the non-perfect is unmarked, and the perfect is formed with have + past participle (e.g., has gone).
The aspect system is the progressive. Non-progressive verbs are unmarked. Progressive ones are formed with have + gerund-participle (e.g., is going).
Moods are usually marked with modal auxiliaries (will, shall, can, may, must) + infinitive (e.g., can go). But the construction of moods in English is complex, and can’t be quickly summarized.
Some grammars will describe a future tense, formed with will/shall, but Huddleston and Pullam argue that there are numerous ways to express future time in English and the will/shall forms are analogous in every respect to the other moods, so this construction of futurity is more accurately described as a mood.