Once I saw a dream

Hindi term (edited): once I had a dream

Here the expression 'to have a dream' means to have an imagination or aspiration.

"Once I had a dream" - may have different meanings:
(1) if it means that there was a time when I used to dream/fancy/desire/wish...etc, then the translation would be (among several other possibilities): कभी मैं सपना देखता था (kabhii main sapnaa dekhtaa thaa), कभी मैं सपना देखा करता था (kabhii main sapnaa dekhaa kartaa thaa),
एक समय था जब मैं सपना देखता था (ek samay thaa jab main sapnaa dekhtaa thaa)....etc. Note that for a female speaker, dekhtii thii, dekhaa kartii thii etc. would be used. Also note that the word khaab may also be used in place of sapnaa.
(2) if the sentence means "on a certain occasion, I dreamt a dream..", then the translation would be (among several other possibilities): एक बार मैं ने एक सपना/ख़्वाब देखा था (ek baar main ne ek sapnaa/khaab dekhaa thaa). Note that dekhaa thaa would remain unchanged even for a female speaker.

dream=सपना/कल्पना

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Note added at 1 day9 hrs (2009-09-27 03:30:23 GMT)
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Mistakenly I wrote मेरी instead of मैंने .

अगर हम अंग्रेजी के वाक्‍य के साधारण अर्थ की बात करे तो तीनों वाक्‍यों का ही प्रयोग हम कर सकते हैं । और यदि हम किसी व्‍यक्ति द्वारा किसी विशेष परिस्थिति के संदर्भ में विचार किए जाने की बात करें तो हम तीसरे वाक्‍य का प्रयोग कर सकते हैं ।