The "a" in spanish

The "a" in spanish
 

Me again.

I'd like to understand this because it causes many errors when work these.
These are a, lo, le, se...
E.g. I should translate this sentence from English to Spanish.
He likes this shirt because it is blue.

I wrote:
Él le gusta esa camisa porque es azul.

Correct version is:
A él esta camisa... etc.

Why the a at the beginnging of the sentence. It was just so good and nearly correct since I did this one often. But I don't understand.
I understand the use of le, lo, se, etc. neither... It is weird. What's the English aquivalent to it?

The 'a' introduces the indirect object. It is always required, no matter who or what the indirect object is.

The other words you questioned are object pronouns. These give most learners of Spanish a lot of grief.
Some of the words you wrote are indirect object pronouns. Some are direct object pronouns. You'll find lots of help in other threads and on the internet if you need to learn more.

In English, the direct and indirect object pronouns are the 'me', 'you', 'him', 'her' and 'it' words.
In German, the direct object pronouns are 'mich', 'dich', 'ihn', 'sie' and 'es'. The indirect object pronouns are 'mir', 'dir', 'ihm', 'ihr' and 'ihm'.

You can find some explanations on how the verb "gustar" works here or here.

Thanks to both of you.

It already gives me bad dreams.

But it's even harder because some of the lessions introducing new times to me at the same time with introducing lo, le, me, se, etc. Now it's not only gusto but gustó or something else. How do you hear the difference when not seeing the sentence? I had so many errors because I don't hear anything that makes a difference.

Listen for which sylllable has the strong stress when speaking.

When pronounced by itself, EVERY Spanish word that has two or more syllables has exactly one stressed (or "tonic") syllable, and all other syllables in that word are unstressed (or "atonic"). The position of the stress within the word is just as important to identifying which word it is as the specific sequence of sounds within the word. Two words that have the same sequence of sounds but with the strong stress on different syllables are different words with different meanings.

You mentioned "gusto" (stressed as "GUS-to") and "gustó" (stressed as "gus-TO"). These two words are conjugated forms of the verb "gustar" (= literally "to be pleasing", usually used to translate "to like"):

gusto (GUS-to) = first person singular present indicative = "I please/am pleasing [someone]" or "[someone] likes me".

gustó (gus-TO) = third-person singular preterite (aka "simple perfect") = "he/she/it/you (formal) pleased [someone]" or "[someone] liked him/her/it/you (formal)".

All regular -ar verbs have the same pattern as gusto/gustó, because:

1. The first person singular present indicative form for ALL regular verbs is marked by the two parts (a) the vowel 'o' added to the stem and (b) the stress on the penultimate syllable (the syllable that preceded the last syllable).

2. The third-person singular preterite form for ALL regular -ar verbs is marked by
the two parts (a) the vowel 'o' added to the stem and (b) the final syllable is the stressed syllable.

Thanks for the explanation!