In your opinion, what is the most common mistake that newbies make? Hopefully, this information will help me avoid it.

Alkanadi:In your opinion, what is the most common mistake that newbies make? Hopefully, this information will help me avoid it.

- trouzo de akuzativo | overuse of the accusative - "mi estas Esperantiston"

- forgesado de akuzativo | forgetting the accusative - "mi havas hundon"

- mistraduko de | mistranslation of EN:"there is" kaj | and EN:"there are" - "there is a bug in my code" -> "tie estas cimo en mia kodo"

http://lernu.net/lernado/gramatiko/demandoj/oe.php

I've been a lingvohelpanto on the lernu courses for a few years now and off the top of my head some of the most common errors I come across are:

Not using the accusative where they should
Using the accusative where they shouldn't
Lack of case/plural agreement between adjectives and nouns
Not using "al" on verbs that traditionally require it
Using "si" as subject
Using -as or -anta for a gerund
Misuse of correlatives (kiu instead of kio, kia instead of kiel, etc)
Using the accusative after a preposition in an invalid way (eg "mi trinkis multe da teon" )
Using "ĝi" when you should use "tio".
Using an adjective when you should use an adverb
General overuse of participles

One that I see even advanced speakers with decades of the language behind them and books published occasionally make is accidentally making a subject that comes after a verb into an object, simply because of familiarity with SVO. This is particularly common with verbs like aperi and manki, which tend naturally to be used as VS.

- Subite aperis viron dum mi promenis

- Mankis tempon por fini la artikolon hodiaŭ

One nearly universal newbie mistake is writing "Englishperanto". (There are probably other mangled versions, but I wouldn't be able to notice them)
--(Note: I believe some of my examples are from your post in the other thread ;P )--
This includes but is not limited to:

- Using o-words in adjectival ways; Ekz. "race horse" -> "vetkuro cxevalo" (in this unfortunate example "horse race" is another valid interpretation...).

- Using the wrong word when there are multiple meanings for the word. One I saw today was "posteno" being used instead of "afisxo" to describe a forum post. This is more of a problem with one-word translations without actual definitions, and I frequently tell people to use the Eo-Eo definition when they are uncertain. I always try to find a good definition here, on the reta vortaro, or on vortaro.net when I come across a word I don't know.

- Directly translating word-for-word from English, including the usual helper words (like the "tie estas" example Clarence666 pointed out). This is what lets me know for sure that the person's native language is English when I see it. Avoid this by trying to think in Esperanto and knowing the grammatical function of the words you're trying to translate.

- As tommjames pointed out it is not correct to translate gerunds into verbs or participles. This stems from the hairy situation in English where we have gerunds, participles, and even adjectives that all end in -ing. Most native English speakers don't really know the difference, so translating it will be hit-or-miss for sure. The good news is "gerunds" in Esperanto do not share an ending with participles. I say "gerunds" because this word means different things for different languages. The simple answer is to use -ado or the -o form of the verb when you are using it as a "thing". ("Mi tre sxatas kuradon" -> "I love running" vs "Mi vidas kurantan hundon" -> "I saw a running dog" )

- The ever-present misuse of accusative (or lack of use). This just takes time and practice in my experience.

- Incorrect usage of correlatives, particularly -o and -u.

I'm probably forgetting something at the moment, but those stick out the most to me. Some of these are only really obvious on people I assume have not studied a foreign language before. For non-english-speaking newbies I sometimes just have a hard time following what they are saying due to errors I must assume are specific to their native tongue.

The best way to break any of these bad habits is to stay involved on the forums (actually posting with some frequency).

• Using "ke" as a correlative (instead of "kiu" or "kio" ) due to to equality in many Indoeuropean languages

Fenris_kcf:

• Using "ke" as a correlative (instead of "kiu" or "kio" ) due to to equality in many Indoeuropean languages

Can you provide an examples? Could you flesh this out more? Do you mean the difference between the conjunction that and the pronouns which and who?

Mi scias, ke mi ne scias!

Estas la hundo kiu manĝis miajn ŝuojn.

Mi mankas al miaj ŝuoj, kio mia hundo manĝis.

parawizard:Can you provide an examples? Could you flesh this out more? Do you mean the difference between the conjunction that and the pronouns which and who?

I think one example might be "It was a man that hit me"
and saying that as "Gxi estis viro ke batis min" instead of "Gxi estis viro kiu batis min."

Hound_of_God:

parawizard:Can you provide an examples? Could you flesh this out more? Do you mean the difference between the conjunction that and the pronouns which and who?

I think one example might be "It was a man that hit me"
and saying that as "Gxi estis viro ke batis min" instead of "Gxi estis viro kiu batis min."

In my opinion it should "It was a man who hit me" in English as well and not using that. It seems to be a gray area historically and I can't really find any definitive rule. Sometimes that is used for the effect, making the person sound less human. In some regions people may take offense to be referred to as "that" instead of who as they feel dehumanized. Though that can be used there, other phrases you could run into problems with using that in place of who. That you can used with restrictive clauses and who can be used with restrictive and non-restrictive.

Restrictive Clause
"All the plant workers who wanted raises went on strike" - Good
"All the plant workers that wanted raises went on strike" - Good

Non-restrictive Clause
"All the plant workers, who wanted raises, went on strike" - Good
"All the plant workers, that wanted raises, went on strike" - Bad

Example take from here
Wiki - Non-Restrictive Clause
Wiki - Restrictive Clause

Either way I would it say it comes down to people just not knowing/remembering about this school of thought. Even if they choose to use that for referring to people in English they will need to remember the ambiguity when translating to other languages. In my opinion I prefer to use the stricter version of who when referring to people unless I am looking for a desired effect.

Now as for French it is stricter as far as that and who, que and qui. There is no ambiguity to be found. You may find some ambiguity in spoken common level french where people pass on the use of the much more complicated pronouns and use que but not qui.

You can find the PMEG table that shows the uses for the different pronouns here: La tabelo

parawizard:In my opinion it should "It was a man who hit me" in English as well and not using that. It seems to be a gray area historically and I can't really find any definitive rule.

Either works, since the question can be "Who hit you?" (-> "It was the man who ...") or "What hit you?" (-> "It was the man that ...")

Fenris_kcf's contribution is a very good one - it's certainly an error I've seen on many occasions. "La hundo, *ke* loĝas en la apuda domo ..." etc.