I think you are mixing up concepts, Isaac. 'Eu vou comer' doesn't need the preposition because it's one of the ways we convey the future in Portuguese: person + verb 'ir' conjugated in the present + main verb in the infinitive. That's the structure for that future and has nothing to do with the main verb in question. It's just a coincidence that in Spanish that structure requires the preposition and that made you think that, as a rule of thumb, Portuguese doesn't use it when Spanish does. On the other hand, when it comes to 'ajudar', it's all about the verb's valency: 'ajudar' needs the preposition 'a' in its structure; you can't leave it out, even in informal language, because it sounds wrong without it (it's not casual to drop it; it's just incorrect and it never happens among natives). The structure (for that sense) is: person + 'ajudar' properly conjugated + another person + 'a' + main verb in the infinitive. Verb valency varies between languages, so that's why it can lead to confusion. I don't know how it works in Spanish, but, for example: 1) In English, we 'listen TO something'; in Portuguese, we 'listen something' ('ouvimos algo'). 2) In English, we 'need something'; in Portuguese, we 'need OF something' ('precisamos DE algo'). 3) In English, we 'try TO do something'; in Portuguese, we 'try do something' ('tentamos fazer algo'). And so on. There's no explanation or correlation between languages; it's just the way they work on their own. Regarding the verb 'ajudar', the preposition is optional in English: we can 'help someone do something' or 'help someone TO do something'; in Portuguese, it is mandatory, as I said before ('ajudar alguém A fazer algo', always).

Don't know about spanish... so I'll try to answer that using the english if that's better than only portuguese :) In portuguese, there are two types of verbs called "transitivo" and "intransitivo" (there is also a third one but it's not important now). The definition of the "verbo intransitivo" is the verb is able to represent completely the action on its own. Like, " O filme acabou. " (the movie ended) you don't need any extra information. You can understand what happened completely. As for the "verbo transitivo", there is a need of extra information. Like, "As crianças precisam de amor". (the kids needs love). if you say only "As crianças precisam", you start to wonder 'what do they need?', so that's why you put "de amor". The teachers here would say for that last sentence "Quem precisa, precisa DE alguma coisa" (who needs, needs something). That 'de' is doing the same as this 'a'. "Eu posso ajudar o senhor A encontrar uma roupa bem bonita para sua namorada" as the teacher would say "Quem ajuda, ajuda A algo" (who helps, helps on something) Those 'de' and 'a' (and others more) are called 'preposições'. As the help needed, leads to another action, you have to use the 'a'. Maybe you'll like that site http://www.soportugues.com.br/secoes/sint/sint11.php

esta correto o uso de A, quer dizer sobre a roupa. el correcto el uso correcto de A, es decir sobre la ropa

esta correto o uso de A, quer dizer sobre a roupa. el correcto el uso correcto de A, es decir sobre la ropa