"I need to pay."
Translation:Yo necesito pagar.
Translation isn't always word for word. If you say "necesito a pagar" It's like saying either "I need to to pay" or "I need to paying" in English.
Infinitive forms (pagar, leer, vivir, etc.) most commonly translate as either to pay, to read, to live, etc. or, in certain specific situations as paying, reading, living.
Here's an article with more detail: https://www.thoughtco.com/infinitive-spanish-basics-3079240
FULL DISCLOSURE: Native English speaker - US, Southern Appalachian dialect. Other uses of English may vary. Advice about Spanish should be taken with a grain of salt.
Because saying "Yo necesito a pagar" would be the same thing as I need to to pay. In any verbs in spanish the ending is the I, WE, YOU, and THEY. for example to run: corro is I run, correr is to run, corremos is we run. The ending is key to who is doing it and or the actual action of doing it (:
Hahaha, I like where you went with that! Sure, you might say it a number of ways, it all depends on the context, e.g. quisiera -'I would like', quiero - 'I want' or in this case, necesito. It's not bad, just a specific context. Like, if a group is all walking out of a restaurant and someone says, 'Oh wait, I (still) need to pay!', or when buying something: 'I need to pay before they'll deliver it'. 'Want' isn't really a fitting verb for some contexts (even when you're not in a hurry to escape!).
Pago is a present tense conjugated form of the infinitive verb 'pagar', and means 'I pay', whereas pagar is not conjugated and means 'to pay'.
So if you have already conjugated 'necesitar' (to need), into 'necesito' (I need), then you don't also conjugate pagar into pago, otherwise you are saying 'I need I pay'.
Basic (but very simplified) rules to remember are, 1). If it ends in 'er', 'ir' or 'ar' it is the unconjugated (infinative) verb form, meaning 'to x', and if it ends in 'o' it is conjugated to mean 'I x', (however these endings are a basic rule as Spanish has a whole bunch of irregular verbs too!) and 2). You don't put two conjugated verbs together in a simple sentence, as my above example shows, it doesn't make sense! :)