अव्यय (Indeclinable) are words that do not change their form regardless of gender, number, or case. They remain constant in all grammatical contexts. There are four main types: 1. क्रिया विशेषण अव्यय (Adverbs — modify verbs): यत्र (where), तत्र (there), कदा (when), तदा (then), अत्र (here), सदा (always). 2. उपसर्ग अव्यय (Prefix particles): प्र, परा, अप, सम्, अनु, अव, निस्, निर्, दुस्, दुर्, वि, आ, नि, अधि, अपि, अति, सु, उत्, अभि, प्रति. 3. निपात अव्यय (Particles): च (and), वा (or), एव (only), ह (indeed), अपि (also), तु (but), हि (because). 4. समुच्चयबोधक अव्यय (Conjunctions): च, वा, अथवा, यदि, तर्हि, अतः.

Important अव्यय for REET with meanings: सदा (always), कदा (when), तदा (then), अधुना (now), श्वः (tomorrow), ह्यः (yesterday), परश्वः (day after tomorrow), एकदा (once), यावत् (as long as), तावत् (that long), यथा (as), तथा (so), बहिः (outside), अन्तः (inside), उपरि (above), अधः (below), पुरा (before), इतस्ततः (here and there), सर्वत्र (everywhere), सर्वदा (always), शनैः (slowly).

REET Exam Tips: Know the classification (4 types) and common अव्यय-meaning pairs. The most common question format is 'identify the अव्यय in a sentence' or 'match अव्यय with meaning'. Remember: अव्यय NEVER change form — that's their defining characteristic.