What is a deductive grammar lesson?
A deductive grammar lesson gives a direct explanation of the grammar structure at the beginning of the lesson.
This is followed by practice with the structure, which should involve some communicative practice at some point in the lesson. It is important to state that the deductive grammar lesson need not be boring or teacher centered -- it can be both interesting and student centered.
In fact, many teachers prefer a deductive lesson because they believe it saves time and helps students focus on what they need to be focused on: The grammar in question.
In a deductive lesson, the teacher can give the students the grammar rule of present perfect and let them explain the rule to each other. This can be done in groups or pairs as the teacher monitors the groups and asks questions to guide them. After the students understand the rule with minimal support from the teacher, they can engage in some practice using the grammar, which can be a correction task, listening task, or production task.
Or, the teacher can explain the rule in a direct way followed by practice using the grammar in various activities such as speaking dialogs, cloze activities, readings with the target grammar examples, writing activities...the list goes on. The point is that practice using the grammar is essential to helping the students add the grammar to their personal language systems.
An important quality of any grammar lesson is that there is a strong communicative component of the lesson. Students need practice using the grammatical structure in both speaking, reading and writing.
Grammar teaching deductively can be very effective. See this website on Presentation, Practice and Production.