Rule 1: 1. Formation of Form III
Insert an alif after the first radical: there is no Form I verb سَفَرَ meaning to travel, but from the noun سَفَرٌ journey Arabic builds سَافَرَ to travel. Perfect فَاعَلَ, imperfect يُفَاعِلُ (prefix u, kasra before the last radical), imperative فَاعِلْ. The imperative needs no hamzat al-waṣl, because the stem already begins with a vowelled consonant: سَاعِدْ help!, not اُسَاعِدْ.
سَافَرَ عَمِّي إِلَى مَكَّةَ فِي الْأُسْبُوعِ الْمَاضِي.
sāfara ʿammī ilā makkata fi l-usbūʿi l-māḍī
My uncle travelled to Mecca last week.
يُسَاعِدُ الْمُسَافِرُونَ بَعْضُهُمْ بَعْضًا فِي الطَّرِيقِ.
yusāʿidu l-musāfirūna baʿḍuhum baʿḍan fi ṭ-ṭarīqi
The travellers help one another on the way.
بَعْضُهُمْ بَعْضًا 'one another': the first بَعْض is nominative in apposition to the subject, the second is the object.
قَابَلْتُ صَدِيقِي فِي الْمَطَارِ.
qābaltu ṣadīqī fi l-maṭāri
I met my friend at the airport.
سَاعِدِ الْمُسَافِرَ الْعَجُوزَ يَا وَلَدُ!
sāʿidi l-musāfira l-ʿajūza yā waladu
Help the old traveller, boy!
Imperative سَاعِدْ; its sukūn becomes kasra before the article.
لَنْ نُغَادِرَ الْمِينَاءَ قَبْلَ الظُّهْرِ.
lan nughādira l-mīnāʾa qabla ẓ-ẓuhri
We will not leave the port before noon.
Subjunctive after لَنْ: the final ḍamma of نُغَادِرُ becomes fatḥa, nothing else changes.
لَمْ يُشَاهِدِ الْأَوْلَادُ الْبَحْرَ قَبْلَ هٰذِهِ الرِّحْلَةِ.
lam yushāhidi l-awlādu l-baḥra qabla hādhihi r-riḥlati
The children had not seen the sea before this journey.
Jussive after لَمْ.