hello, actually, I agree with Aya in something, arabs have no difficutlies to read the arabic withoout the marks of vowels.
Now, lots of poeple were asking the same question, and I would like to explain that to you too, and if you have more question feel free to ask. I don't know what level you are in arabic, but I will answer in easy way to understand. I guess you know how to read the arabic with the vowel marks. and that's really great !
the vowel marks are important for every learner who is interested to the classical or the standard Arabic, and it's the key to know grammar well. Lots of grammar rules are related to the final mark of vowel and other change some marks to have other sense, but all that are with grammar or without grammar. So no worries.
The answer :
in arabic we have two parts of speech or reading as you know. every word has a weight (وَزْنٌ)
we have two parts :
الأَوْزَانُ السَّمَاعِيَةُ [ the listening weights]:
this part should be memorized by heart, like the name of people, the name of the places, the countries, the things the animals...... they don't change their marks of vowels expect the final mark which can be changed, it dépends to its situation the sentence[ the suject, the object...]
قَمَر يُوسُف أَسَد......
الأَوْزَانُ الْقِيَاسِيَةُ [ the standard weights]
this part is related to grammar, and on it, we can create other words and we can know the pronunciation of the word without having the marks, because we knew before the weight of them is the standard one or the origin one to make other words.
for example, making the suject, who does the verb we use the standard weight [ فَاعِلٌ], with every 3th verb like : كَتَبَ، قَرَأَ، أَكَلَ
فَاعِلٌ كَاتِبٌ قَارِئٌ آكِلٌ
كاتب قارئ آكل
and it has a lot. they also change their final mark, because it dépends to its place in the sentence.
So my advice is practicing. If you wanna help you with that, I can help with practice, but I accept only the surious people. I have teached the standard Arabic since a long time, but doesn't mean I am a teacher or a major in Arabic :).