A quick family-tree refresher
The Dravidian family has four major literary branches. Tamil and Malayalam belong to the southern branch and split from a common ancestor relatively recently in linguistic terms. Kannada belongs to the south-central or "Kannada-Tamil" branch depending on how linguists draw the lines. Telugu is south-central but distinct — it broke off earlier and absorbed more Sanskrit influence over the centuries.
That tree predicts roughly what you find on the ground. Tamil and Malayalam speakers understand far more of each other than either understands of Kannada. Kannada speakers find Tamil structurally familiar but vocabulary often opaque. Telugu sits a bit apart from all three.
What actually transfers between Kannada and Tamil
Core deep vocabulary — body parts, basic family terms, numbers, common verbs — has clear cognate pairs. Eye is kannu in Kannada, kann in Tamil. House is mane versus manai. To come is baruvudu / baa versus varuvadhu / vaa. The roots are visible if you look for them.
Sentence grammar is structurally near-identical. Subject-object-verb order, postpositions, verb agreement with subject, the same broad approach to negation. Once you have one Dravidian language, the second's syntax feels familiar even when the surface words are different.
Politeness particles and address forms map across reasonably well. The instinct to add a respectful plural ending for elders or strangers is the same. Specific forms differ, but the social grammar is the same.
Where it breaks: everyday vocabulary often does not transfer cleanly. Tamil has fiercely preserved its Dravidian-origin words for many concepts where Kannada has adopted the Sanskrit alternative as the everyday term. The Tamil for "time" is neram; the everyday Kannada is samaya (Sanskrit). The Tamil for "thank you" is the construction nandri; in casual Kannada you'll most often hear thanks from English or dhanyavaada from Sanskrit. Tamil speakers learning Kannada often find that their assumption of cognates fails specifically in the most useful vocabulary.
What transfers between Kannada and Telugu
More Sanskrit overlap than with Tamil. Because Telugu absorbed Sanskrit vocabulary heavily over centuries — much more than Tamil did — Telugu speakers learning Kannada find the formal and academic vocabulary almost immediately familiar. Samaya, samasya, satya, sahaaya: same words, near-identical pronunciation in both languages.
Grammar overlap is high but not as high as you'd hope. Telugu and Kannada share Dravidian sentence structure, but verb morphology — the actual endings on verbs across tense, person, and number — diverged significantly. Telugu speakers tend to find Kannada verb forms a little surprising even when sentence shape is familiar.
Many everyday Dravidian words are different. Water in Telugu is neellu; in Kannada neeru. Sleep is nidura in Telugu, nidde in Kannada. Recognisable in a careful read but easy to mishear in conversation, especially at the speed Bengaluru actually speaks.
The pattern most people miss
Cognates are easiest to spot in formal vocabulary and hardest to spot in everyday vocabulary. This is the opposite of what learners expect. You walk in thinking your existing language will help you order tea and ask for directions; instead it helps you discuss culture and government, which you weren't planning to do this week.
The practical implication: do not skip the basic vocabulary work just because you already speak a Dravidian language. The everyday Dravidian-origin words diverge more than the Sanskrit-origin layer, and the everyday words are the ones you need first.
A rough estimate of head start
For a Tamil speaker learning Kannada, my rough estimate based on conversations with bilingual friends is about 30–40% time saved over a learner with no Indian-language background — concentrated mostly in grammar intuition rather than vocabulary. For a Telugu speaker, more like 40–50%, with the gain spread more evenly across vocabulary and grammar. For a Hindi speaker, around 25–35%, almost entirely in Sanskrit-origin vocabulary and sentence rhythm.
None of these numbers means you can skip the basics. They mean the curve is gentler in the right hands. If you're learning Kannada with prior Dravidian exposure, don't waste the advantage by trying to translate word-for-word — let your instinct for grammar guide sentence construction, and learn the everyday vocabulary fresh.