For storyline, Yes you have to play HL1 first to get a grip on HL2. It's a continuation of the first Half Life game & you won't be able to relate to the characters if you play HL2 first (like me). Else you could read the whole story over Wikipedia & bounce off to play Hl2.
for story-wise it's recommended to play HL1 before HL2.. it will be more immersive than reading the story elsewhere.... though HL1 graphics is decade old, its story and gameplay is gr8... if you dont have time to play HL1, just read the story and dive into HL2 you will still have a good experience
(Disclosure: I'm a HL Fan )
Portal is a completely different game... no need to play HL1/2 beforehand
If you want to get an idea of what's going on and why, then I'd say you go and play HL1 first. If possible, get the revamped HL: Source version; it looks much better than the original HL.
Portal is a completely different game, for which you need not play HL first.
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