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The causes of a sore or painful tongue are similar to those of a sore mouth or throat. The cause is usually obvious upon examination but there are some obscure causes. Investigations may include an FBE, serum vitamin B12, folate and ferritin, a swab or a biopsy of a suspicious lesion.
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Look for evidence of trauma, esp. from a sharp tooth
A miserable child with a painful mouth and tongue is likely to have acute primary herpetic gingivostomatitis
In your history take note of self-medications, esp. sucking aspirin, a history of skin lesions (e.g. lichen planus) and consider underlying diabetes or immunosuppression
A long history of soreness with spicy or other foods indicates benign migratory glossitis (geographical tongue) or median rhomboid glossitis
Any non-healing or chronic ulcer requires urgent referral
Glossodynia (painful tongue) characteristically presents as burning pain on the tip of the tongue—it can be a real ‘heartsink’ presentation; consider depressive illness as an underlying cause
Macroglossia (large tongue): consider acromegaly, myxoedema, amyloidosis, lymphangioma
Strawberry tongue: consider scarlet fever, Kawasaki disease
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Geographical tongue, 
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BLACK OR HAIRY TONGUE
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This causes bad tastes and a malodorous oral cavity. Basically a harmless condition that can be related to smoking, poor oral hygiene or use of antibiotics.
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Brush with toothbrush using sodium bicarbonate paste or
Suck fresh pineapple pieces: cut a thin slice into eight segments—slowly suck a segment on the back of the tongue for 40 secs, then slowly chew it